Reading: The Age of the Old Catholic Fathers [continued] (The Early Church, Chapter 8)

In Chapter 8, we examine in more detail the emergence of the idea of a catholic or universal church, a church which had a profound unity with all churches over the face of the earth. In this chapter, it is not important to learn the name of all the old church fathers, but to see the overall flow of the growth of doctrine as the church developed.

The Early Church by James Orr


THE AGE OF THE OLD CATHOLIC FATHERS - (Continued). (A.D. 180-260) 

The chief interest of the period whose external history and internal development we have sought to describe is connected with the names of its great teachers. These form a galaxy of rare brilliance. The study of their works is at the same time the study of the theology and literature of the age. 

Irenaeus of Gaul

The personal information we have of this great Father are scanty. He was born about A.D. 120, perhaps a little later; was a native of Asia Minor; in early life was a disciple op Polycarp, the disciple of St. John. In an epistle to his fellow-pupil Flomius, who had lapsed into Gnosticism, he speaks of the vivid recollection he retained of Polycarp's discourses, and how they agreed with what was related in the Scriptures. He was a presbyter in Lyons during the persecution under Marcus Aurelius in A.D. 177. The Montanist controversy was raging, and Irenaeus bore an intercessory letter on behalf of the Montanists from the martyrs to Eleutherus, the Bishop of Rome. 

After the martyrdom of the aged Pothinus, Irenaeus, as the fittest man, was chosen bishop in his place. The only other occasion on which he comes into view is a few years later (A.D. 190-94) in connection with the action of Victor of Rome in the Quarto-Deciman controversy (see below). The date of his death is uncertain (A.D. 202-3?). All through Irenaeus showed himself a man of peaceful and conciliatory spirit in marked agreement, Eusebius says, with his name (= peaceful). 

His one literary monument (besides fragments) is his great work, in five books, Against Heresies, directed specially against the Valentinians (A.D. 180-190). It exists only in an early Latin translation; portions of the Greek, however, are preserved by other writers. The author's theological opinions are developed incidentally, but sufficiently to show that Irenaeus had a theology of a very definite and organic character. The central thought in his conception of Christianity is the incarnation. Creation needs the incarnation for its perfecting. Only through the entrance of the Word (Logos) into humanity could man be led to his destination as a son of God. Irenaeus has no doubt as to the eternal existence of the Word.  The Son has always existed with God, has always revealed the Father, has always revealed the full Godhead. Redemption is brought under his favorite idea of a recapitulation of humanity in Christ. Christ is the compendium of the race; sums up the nature, the experiences, and the history of mankind in Himself. His obedience retracts the disobedience of the Fall. As our Head he wins for us a complete victory over Satan. He enters into our lot and doom as sinners, and ransoms us by His death. A trace only is discernible of the theory afterwards developed that Satan through the Fall obtained rights over men which had to be respected. In eschatology Irenaeus is crudely Chiliastic, meaning he had a simple view of the 1000 year reign of Christ foretold in Rev. 20.

The sacramental teaching of Irenaeus conforms to the well-established Catholic type. The Eucharistic elements, he writes, are "antitypes" of the Lord's body and blood; yet there is a real mystical union of these elements with the body and blood of Christ, so that in receiving them the communicant is nourished by the latter. 

Tertullian of Carthage

Tertullian is the first of the great Latin Fathers, and founder of Latin theology. His general place in the history is about twenty years after Irenaeus. He follows Irenaeus closely in his antignostic polemic and doctrine of the Church. The two men, however, are as different as be conceived. The calm, temperate spirit of Irenaeus bears no resemblance to the fiery, impetuous nature of the North African Father. No impartial person will doubt his deep or sincere piety; yet the fire within him burned often with a murky flame.

Tertullian was born at Carthage probably about A.D. 160. His father is said to have been a proconsular centurion, and he was educated for the law. His life till manhood was spent in heathenism, but its follies and pleasures left his soul unsatisfied. His conversion to Christianity may have been about A.D. 192. He probably became a presbyter of the Church at Carthage. We know that he was married, and that his wife also was a Christian. 

The decisive event in his career was his conversion to Montanism (c. A.D. 202). Thereafter his relations with the Church were embittered, and he withdrew from its communion (Against Praxeas 1). It is doubtful, however, how far this withdrawal went. It is certain that Tertullian always regarded himself as belonging in a true sense to the Catholic Church, and there are evidences that towards the end of his life the harshness softened. His death is placed A.D. 220- 40. Whatever his faults of temperament, Tertullian's ability as a Christian advocate is second to none. 

His literary activity was prodigious. His pages sparkle with brilliant and original thoughts; are, indeed, for vigor, terseness and mastery of literary expression unsurpassed in patristic literature. Cyprian's admiration of him was such that it is said a day never passed without his calling for some of his works, saying, "Give me the master." His writings are usually divided into those written before and those written after he became a Montanist, though it is doubtful to which class some are to be referred. 

The first period (A.D. 197-202) includes the tract To the Martyrs (A.D. 197), and the Apology (A.D. 198-99). To the Nations and the beautiful tract On the Witness of the Soul (the germ of which lies in "the soul naturally Christian" of the Apology, ch 17) are related to the Apology.  A number of short treatises (happily) called Tracts for the Times, deal with questions arising out of the life of the time, and with practical subjects, e.g., on “The Spectacles; on “Idolatry”; on the “Attire of Women;” two treatises “To my Wife”, discussing second marriage ; and on “Penitence, Prayer, Patience.”

These shorter pieces especially exhibit a mixture of argument, wit, sarcasm, and teasing very characteristic of Tertullian. Though not yet a Montanist, his standard of judgment is always severe. 

The second period comes after A.D. 202. This period reflects his changed attitude to the Church, and shows Tertullian at his best and his worst. The resources of his rhetoric, his brilliant antitheses, his Christian zeal, his powerful and often convincing reasoning, command admiration. On the other hand, his faults of temper and argument are often glaring. Here, again, we have to distinguish between his shorter occasional pieces which were called forth by special circumstances. Examples are on “The Soldier's Crown,” on “Flight from Persecution,” “The Veiling of Virgins,” on “Single Marriage,” on “Fasting,” and so forth.  His longer controversial works are of a different nature. The principal of these are his great work, in five books, Against Marcion, and his treatise Against Praxeas. Reference should be made also to his forcible tractate To Scapula (the proconsul), in which, A.D. 212, he powerfully champions the cause of the whole of the Christians. 

Tertullian's abiding services to the Church are those which he rendered as apologist and theologian. The Apology of Tertullian is regarded as his masterpiece. It is addressed to the emperor, and is a noble piece of pleading. The opening chapters are introductory; they urge that Christianity is hated because it is unknown. The body of the Apology is divided into two parts. The first refutes the charges against the Christians, such as the popular charges of killing infants and practicing incest in their assemblies, then the capital charges of irreligion and disloyalty to the emperor. The second part describes in beautiful words the simple, spiritual, and orderly character of the Christian worship, and the real nature of the much-maligned love-feast. The closing portion replies to objectors, and reminds of coming judgment. 

As a theologian Tertullian left his deep stamp on later thinking. He practically created the Latin ecclesiastical tongue, and gave to theology many of the terms which have become its permanent possession. These include one substance, three persons, satisfaction, merit, New Testament, and rule of faith. On the Trinity he followed the views of the apologists in not attributing to the Son an eternal personal existence. The Trinity is an internal Divine "economy”, with a view to creation and redemption. He follows Irenaeus pretty closely on the doctrines of Man and the Incarnation. Man was made after the image of the future Incarnate. The earlier appearances of the Son to the patriarchs are "rehearsals” of the Incarnation. Tertullian has a much deeper view of sin than obtained in the Greek Church; but his ideas of penitential satisfaction obscure grace, and give a gloomy tinge to his theology. The words, "This is My body” in the Supper are explained, "This is the figure of My body”; but a real presence in the elements is presupposed. 

The Alexandrian School

Pantaenus and Clement

Alexandria was, next to Athens, the city of the Greek world in which intellectual tendencies of every sort met and commingled. It was to be expected, therefore, that early in Church history, in this busy center the attempt would be made to unite Christianity with what was best in the thought and culture of the time. This, accordingly, is what we see taking place in the famous Catechetical School at Alexandria.

It is characteristic of the Alexandrian School that it takes a genial attitude to heathen learning and culture. The school regards Greek philosophy and science as in its way also a providential preparation for the Gospels. It seeks to meet an antichristian Gnosis by a better Gnosis, which grows out of faith and love. It is speculative, liberal, and idealistic in spirit.  In its Scriptural methods, the school is allegorical, though not to the subversion of the history, as in the heretical Gnosticism. 

We know very little about the first major teacher of this school, Pantaenus,(c. A.D. 180). He was a Stoic philosopher, well trained in Greek learning, and the first, Origen says, who applied this learning in Christian instruction. His school was designed for catechumens, that is, those in training for baptism. But many heathens who desired instruction attended. Either before or after his catechetical labors he travelled widely in the East as an evangelist, penetrating as far as India. There, it is said, he found a copy of the Gospel of Matthew (in Hebrew), which had been left by St. Bartholomew. 

His most distinguished pupil was Clement, who succeeded him as head of the school in A.D. 189. Clement of Alexandria was born, probably at Athens, A.D. 150-60. Brought up in paganism -- he speaks even of his initiation into the mysteries -- he undertook a series of travels in pursuit of truth, but found no rest till he met with Pantaenus. That "Sicilian bee," he says, "gathering the spoil of the flowers of the prophetic and apostolic meadow," engendered in his soul a deathless element of knowledge. His own genius gave new luster to the school, over which he presided for thirteen or fourteen years, till the persecution of Severus (A.D. 202) compelled his withdrawal. After 202, Clement is virtually lost sight of. He is supposed to have died about A.D. 220. Throughout he may be regarded as contemporary with Tertullian. 

Clement's genius is cast in a mold totally different from that of the other Fathers we have named. He was, like Tertullian, a man of amazing learning, but he applied his learning in quite another way. He has none of the austerity of Tertullian. Instead Clement was soaring, poetic, idealistic, sympathetic to other schools of thought in his views of truth. On the other hand, his power of reducing his ideas to logical order and connection is limited. His thought loves to roam free and unfettered, and his style in writing is exuberant and discursive. 

Of the known works of Clement we are fortunate in possessing the three greatest -- which form one major work. They belong to the period of his work in Alexandria, and give a good idea of his instruction. The first of these three integrated works is The Address to the Greeks which aims at conversion from paganism. The second is Paedagogus or Tutor. This is a manual of moral discipline which enters into minute details of conduct. The third book is entitled The Stromata or Miscellanies. Here Clement aims at initiating his catechumens into the higher knowledge. These follow, he tells us, the method of the all-glorious Word, who first addresses, then trains, and finally teaches. The Word is the "Tutor". The Stromata, while dealing largely with the relations of faith and knowledge, do not give much help in apprehending Clement's theology. If we possessed his Outlines, a work lost to history, we might have been in better position to describe his theological outlook. 

The central idea is the Logos (Word) as the enlightening source of all truth in humanity. The Logos is eternal, but the Trinitarian distinctions are so idealistically conceived as almost to lose their personal character. Even the sacraments are apprehended in a highly ideal way. Clement prepares for Origen by teaching a preaching in Hades for those who died without opportunity of repentance in this life, as well as for the righteous through the law and philosophy, i.e., just men, both Jews and Gentiles, who died before the Advent. 


 Origen was the favorite pupil of Clement, as Clement had been the disciple of Pantaenus. We recognize in him the greatest of the teachers of the early Church, in fact, one of the greatest minds the Church has seen in any age.

Origen was born at Alexandria in A.D. 185. His parents were both Christians. He showed remarkable ability as a boy, committing to memory large portions of Scripture, and often perplexing his father, Leonidas, by the questions he asked. His father corrected him, but in secret thanked God for such a son. Often, while Origen slept, his father kissed his breast as a temple of the Holy Ghost. When the persecution broke out in A.D. 202 his father was one of the first victims. Origen labored to support the family, and managed to collect a small library. His reputation was such that, on the withdrawal of Clement, he was convinced, though only a youth of eighteen, to take the oversight of the school of Clement and give instruction in it (A.D. 203). The persecution still raged, and many of his early pupils suffered martyrdom. Origen, however, was undaunted, and his labors were crowned with remarkable success.

To maintain his life at a subsistence level, as he would receive no payment, he sold his valuable collection of classical books. He went further, and taking literally the injunction in Matthew 19. 12, he performed an act of self-mutilation, which he lived bitterly to regret. In order better to qualify himself for his work, he took lessons in philosophy from Ammonius Saccas, the founder of the Neo-Platonic school. He learned Hebrew also to prepare him for his Biblical studies. His course embraced arts and letters as well as studies properly theological. These preparatory studies he subsequently handed over to a colleague.

His period of labor in Alexandria lasted for twenty-eight years (A.D. 203-31). It was broken by visits to Palestine, in the first of which (A.D. 215-18) he taught in the churches; in the second during which his travels extended to Achaia, (A.D. 228-31), he was ordained presbyter. These steps drew down on him the displeasure of the narrow-minded bishop Demetrius, and compelled his departure from Alexandria. A council convened by the bishop excommunicated and deposed him (A.D. 231). The bishops in Palestine and elsewhere treated this sentence as null. 

The second period of his work was at Caesarea, where he opened a school on a still larger scale, and conducted it with even more brilliant success. His labors at Caesarea, broken only by a brief withdrawal during the persecution of Maximin (A.D. 236), continued for nineteen years (A.D. 231-50). Origen was apprehended, imprisoned and tortured in the persecution of Decius (A.D. 250). He was released in A.D. 251, but died from the effects of the torture in A.D. 253 (254?), at the age of sixty-nine. 

We can give only an indication of this Father's extraordinary literary labors. During his later residence at Alexandria he wrote many of his Commentaries, and also his book on First Principles -- our first work on systematic theology. A wealthy layman, Ambrose, provided him with the means of carrying on his labors on the most extended scale, providing Origen with shorthand writers, and such. A colossal work, which occupied him for twenty-eight years, was his Hexapla a collation of the Septuagint with the Hebrew text, and three other Greek versions (the Hebrew being printed also in Greek letters as a sixth column). 

The work, except the Septuagint part, has perished. To Caesarea belong Homilies, treatises on Prayer and Martyrdom among others. In A.D. 249, in the reign of Philip, he wrote his great work in eight books, Against Celsus-- the noblest apology of the early Church. 

It has already been hinted that his expositions of Scripture give large scope to the allegorical method. As a theologian Origen shows an unequalled speculative genius.  He distinguishes between what belongs to the rule of faith and points which the doctrine of the Church leaves undetermined. To his credit, on these issues Origen claims for his speculations only tentative and provisional value. 

He emphasizes in the Trinity the "eternal generation" of the Son; on the other hand, lays such stress on the hypostatic distinction, and subordination of Son and Spirit to the Father, as almost to dissolve the Divine unity. He speaks even of the Son in relation to the Father (absolute deity) as "a second God." As God, he thinks, must eternally have worlds on which to display His omnipotence, he teaches eternal creation. There is a pre-existence of souls, and sin is explained by a fall of souls in this pre-existent state. There was one pure soul that did not fall, but clave in love to the Logos. This is the soul of Jesus. In this way, Origen explains the sinlessness of Christ. 

Redemption he regards under many points of view --among them that of a deception of Satan, who cannot retain the soul of Jesus, given him as ransom price for men. Origen is the first pronounced restitutionist in the Church. All souls and worlds, he thinks, will yet be brought back to God. The daringness of some of these speculations involved the Church in much trouble in the years to come. 

Apart from his theological views, Origen is a valuable witness to Christian practice. He bears witness, for instance, to the usage in the Church of infant baptism, and traces the custom back to the apostles. Tertullian, on the other hand, advised delay. 

The Church of Rome in this Period

Hippolytus and Callistas 

Many circumstances combined to exalt the Church of Rome in the second century to a position of exceptional pre-eminence. The circumstances included the fact that Rome was the political capital. The church was ancient and associated with the Apostle Peter.   Further, the church itself was wealthy and gave generously to other churches. This pre-eminence was, however, solely one of respect and honor. It did not mean that the Church of Rome was as yet allowed any real authority or jurisdiction over other Churches. The aim of the bishops of Rome, on the other hand, was to change this position of honor into one of actual authority. Every claim of this kind was strenuously resisted by other bishops. 

A case which makes this clear, and at the same time marks a stage in the claims of the Roman bishop, is that known as the Quartodeciman controversy or dispute about the time of keeping Easter. In Asia Minor the Churches began and finished their celebration on one day -- the fourteenth day of Nisan, or day of the Jewish Passover, on whatever day of the week it might fall. They held that this was the custom handed down to them from the apostle John. Rome and the Churches of the West, on the other hand, followed not the day of the month but the day of the week. They began on Friday of the Passover-week (Good Friday) and ended on the Easter Sunday morning. The matter was discussed in a friendly spirit between Polycarp, of Smyrna, and Anicetus, Bishop of Rome (c. A.D. 155), without, however, a settlement being arrived at. 

It was the occasion of a sharp controversy in Asia Minor itself between Melito of Sardis and Apollinaris of Hierapolis (c. A.D.170). Melito defended the Asiatic practice. But the most important stage in the controversy was in A.D.190-94, when Victor, a haughty and imperious man, was bishop of Rome. Victor issued a mandate requiring conformity to the Roman practice; then, when protest was made, threatened the excommunication of the Asiatics. This assumption of authority was too much even for many who agreed with Victor in principle, and immediate protests were made. The chief of these was from Irenaeus, who, in a letter to Victor, earnestly reproves him for his arrogance. Irenaeus was successful in his protest, and the excommunication was not carried out. The Roman custom was ultimately affirmed at the Council of Nicaea (A.D. 325), though not till it had become generally accepted throughout the Churches. 

The bishops next in succession to Victor were Zephyrinus (A.D. 200-18) and Callistus (A.D. 218-23), regarding whom there is a curious story to tell which is best connected with the account of another great Church Father -- Hippolytus.

Hippolytus has had a most singular fate. A voluminous and learned writer, and one of the most conspicuous figures in the Roman Church of his day, he seems afterwards to have dropped almost entirely out of view. Two interesting discoveries have restored him to our knowledge. First, his statue was dug up in Rome in 1551. On the back of the chair were his Easter cycle and a list of his writings; and second, in 1842, his long-lost work, in ten books, A Refutation of All Heresies was recovered and published. The first book had long been attributed to Origen, the second and third books are missing from the MSS., but the rest of the work is nearly complete. A valuable feature in the book is the original light it throws on the system of Basilides. But by far its most interesting service is its account of the state of the Roman Church under the two bishops Zephyrinus and Callistus, and of Hippolytus' own relation to them. 

Hippolytus in early life was a helper of Irenaeus in Gaul or Rome. Later he headed a party of opposition in Rome to the bishops Zephyrinus and Callistus, whom he accuses at once of doctrinal heresy and of scandalous laxity in discipline. Zephyrinus he describes as a weak and illiterate man, covetous and accessible to bribes, and in the latter part of his life completely under the influence of Callistus. The latter used him for his own purposes, and among other things inclined him to the adoption of the Patripassian heresy, which was then being actively disseminated in Rome. 

 The account of Callistus is in the highest degree unfavorable. Originally the slave of a Christian master, he embezzled the funds of a banking business; fled, and, when about to be captured, tried to commit suicide. He was sent to the house of correction. Later, for a disturbance in the Jewish synagogue, he was banished to the Sardinian mines. We next find him as an associate of Zephyrinus, who set him over the cemetery ever since called by his name. On the death of Zephyrinus, he had influence enough to get himself appointed as bishop in his place. His scandalous administration is pictured in the darkest colors by Hippolytus. 

The difficulty is to know what position precisely Hippolytus himself occupied. He assumes the office of bishop and withholds that designation from Callistus. Hippolytus speaks of Callistus only as head of a school. 

Hippolytus was probably a rival bishop to Callistus, set up by his own followers -- the first of the long line of anti-popes. Yet, the Church later canonized him as a saint! 

The remaining fact of his life of which we can speak with certainty is that he and the bishop Pontianus were transported to Sardinia in the persecution of Maximin (A.D. 235). Some kind of reconciliation must have taken place, for the bodies of both were brought back to Rome about A.D. 236-37, and deposited in their respective sepulchers on the same day. 

Cyprian of Carthage

Completion of Idea of Old Catholic Church

Cyprian is the last of the old Catholic Fathers, and he marks the transition to the next period. Cyprian is not great as a theologian, but he is a great churchman. To him belongs the distinction of having placed the capstone on the organization of the old Catholic Church which we have seen being built up by many hands from the days of Ignatius. His personal history presents us with a career of splendid self-sacrifice. 

Cyprian was born at Carthage, about A.D. 200, of noble and wealthy parents. Previous to his conversion he was distinguished as a teacher of rhetoric. He was won to Christ about A.D. 245 through the instrumentality of an aged presbyter, Caecilius, who directed him to the study of the Bible. Cyprian gave proof at once of the thoroughness and decision of his profession by taking Christ's command literally, and voluntarily selling his fine estate for the benefit of the poor. Baptism followed rapidly on conversion, and was commemorated by his adoption of the name of his spiritual father, Caecilius. In a writing of this period, To Donatus, Cyprian gives a beautiful description of the effects of his conversion, and of the contrast between Christianity and heathenism in a moral respect. 

 He was shortly after ordained a presbyter, and a little later -- only two years after his baptism -- was raised by popular acclamation to the dignity of bishop. His elevation gave deep offence to the presbyters who had been passed over. Five presbyters objected to his ordination. The jealousy of these five created most of his troubles later in life. Thus at the very beginning of his Christian journey, Cyprian found himself at the head of the clergy of North Africa. 

In A.D. 250 the storm of the persecution under Emperor Decius broke over the Church. Cyprian thought it prudent to withdraw for a time that he might better direct the affairs of the Church, and prevent it from being deprived of its head. We will discuss the difficulties this raised for Cyprian later. 

Cyprian returned to Carthage in A.D. 251, when the persecution had ended through the death of the emperor. In A.D. 252 came the great pestilence, which afforded opportunity for a display of Christian devotion and charity such as paganism was incapable of performing. A scheme was drawn up for the systematic visitation of the city; a ministry of help was organized; some undertook the work of nursing and burial; and through their unremitting efforts a general pestilence was averted. 

Under the Valerian persecution, A.D. 257, Cyprian was banished to a city some forty miles distant. A year later (A.D. 258) a more severe edict was issued, and he was sentenced to death by beheading. The martyrdom took place on a level plain near the city in presence of a vast concourse of spectators, all of whom, even the pagans, did him reverence. 

Cyprian, as said above, was less a theologian than a great church leader. The trying circumstances in which he was placed, and the oppositions he had to encounter, forced on him the task of strengthening to the utmost the bonds of church unity, and of seeking, in argument with his opponents, a dogmatic basis for that unity. The chief works in which this basis is set forth are his eighty -one Epistles (some of which were not actually composed by him), and, above all, his treatise on The Unity of the Church

Cyprian's doctrine of the Church may be summed up in three points. (1) The unity of the Church as represented by the episcopate (the hierarchy of bishops). Cyprian gives this a new grounding in basing it on the promise of Christ to St. Peter (Matt. 16. 18, 19). Peter, however, only represents the unity of the Church in a symbolical way. It is not the bishop of Rome only, but the whole body of the episcopate, which inherits Peter's prerogatives. (2) The priesthood of the clergy. Cyprian is the first to give this conception fixed and definite shape. The development of the idea of sacramental grace had been growing for some time already, especially of the Eucharist as a sacrifice. The sacrifice in the Eucharist was originally the spiritual sacrifice of prayer and thanksgiving, or the offering up of the worshipper himself. The idea was extended to the gifts from which the elements of the Supper were taken; then to the elements. 

Once the idea was established of a real mystical presence of the Lord's body and blood in the elements, it was natural that the conception of the sacrifice should change. The Sacrament becomes a real offering up of the body and blood of the Lord -- a renewal of the sacrifice on the Cross. Thus the idea of the sacrifice as a sin-offering and of the priest as one who offers the sacrifice at the altar (in the Jewish and pagan sense), becomes established in the Church. The clergy are a priestly class, mediating between the people and God, and conveying grace to the people from God. The distinction of clergy and laity becomes absolute. (3) With all this Cyprian held firmly the autonomy of each bishop in his own Church. He resisted all arrogant pretensions on the part of the bishop of Rome. On the question of the re-baptism of heretics, e.g, he came into violent collision with Stephen of Rome (A.D. 255-56), who wished to impose his own views on the Churches of North Africa. The Pope's unqualified primacy gets little help from the Fathers of this age. 

 All this points us toward the conclusion which Cyprian boldly draws, that outside of this visible, organized Church there can be no salvation. "Extra ecclesiam nulla salus.”  Hence schism is the worst of sins; excommunication dooms the soul to being lost. 


Last modified: Thursday, August 9, 2018, 1:10 PM