Tatian, a Christian apologist born early in the 2d century (110, Zahn), was an Assyrian by birth, studied Greek philosophy, and wandered as a sophist round the Roman world, but about 150 at Rome was won to Christianity by the simple charm of the Old Testament Scriptures and the example of the purity and courage of the Christians. He became a disciple of Justin, in whose lifetime he wrote his Oratio ad Græcos (ed. by Otto, Jena, 1851; by Schwartz, Leip. 1888), a glowing and uncompromising exposure of the faults of heathenism as compared with the new 'barbarian philosophy.' After Justin's death (166) Tatian fell into evil repute for heresies, and he retired to Mesopotamia, probably Edessa, writing with characteristic fearlessness and vigour treatise after treatise, all of which have perished. He was certainly infected with Gnostic notions of the universe, the supreme God, the demiurge, and the world of æons; but the notions of which gave most offence were his excessive asceticism, his rejection of marriage and animal food, and adoption of the practices of the Encratites. His name became a synonym for all manner of dangerous and unsettling tendencies, and his doctrines were assailed in turn by Irenæus, Tertullian, Hippolytus, Clement of Alexandria, and Origen. Neither the place nor date of his death are known, but it took place perhaps at Edessa, and probably about 180. Of his writings one maintained a place of importance in the Syrian Church for two hundred years, and supplies one of the most interesting chapters in the history of sacred literature. This was the Diatessaron, a gospel freely constructed out of the four gospels known to us, not a harmony in the modern sense, but a kind of patchwork gospel, its principle amalgamation not comparison in Lightfoot's phrase. This Harnack thinks was written in Greek, but Zahn and most other scholars, among them Lagarde, Bâthgen, Lightfoot, and Hilgenfeld, in Syriac. There is no mention of the Diatessaron in any Latin writer before the middle of the 6th century, when Victor of Capua attributes an anonymous gospel harmony he had found to Tatian, neither Cassiodorus, Gelasius, Jerome, Augustine, or Juvenus so much as naming it. Of Greek writers the first to mention it is Eusebius: 'Tatian composed a sort of connexion and compilation, I know not how, of the gospels: and called it τὸ διὰ τεσσάρων. This work is current in some quarters even to the present day.' Zahn thinks from this that Eusebius had never himself seen it, but, knowing Tatian to have been a heretic, thought his compilation a mere distorted gospel like that of Marcion and Gnostic teachers. Epiphanius says, 'the Diatessaron gospel is said to have been composed by him [Tatian]. It is called by some the gospel according to the Hebrews.' But on Syrian soil in Theodoret, bishop of Cyrrhus, near the Euphrates, we first find (453) an explicit account of the book: 'He [Tatian] composed the gospel which is called Diatessaron, cutting out the genealogies and such other passages as show the Lord to have been born of the seed of David after the flesh. This work was in use not only among persons belonging to his sect, but also among those who follow the apostolic doctrine, as they did not perceive the mischief of the composition, but used the book in all simplicity on account of its brevity. And I myself found more than two hundred such copies held in respect in the churches in our parts. All these I collected and put away, and I replaced them by the gospels of the four evangelists.' Thus we see that in the 5th century this work was used in the Syrian churches as the form in which the gospel was read, and further back still we find evidence of its use in the 3d-century Doctrine of Addai (Cureton's Ancient Syriac Documents, 1864, and G. Phillips, Syr. and Eng. ed. 1876), and in the Homilies of Aphraates (fl. 340) and the famous Ephraem Syrus. The Syriac text of the Homilies of the former was edited by Professor Wright in 1869, and Zahn has proved that the key to the difficulty of his gospel citations is the fact that he used the Diatessaron. Bar-Salibi, a Syrian bishop (12th cent.), distinctly states that Ephraem wrote an exposition of Tatian's Diatessaron. Lightfoot printed his famous article on Tatian (Cont. Rev. May 1877), ignorant of the fact that a year before Dr Moesinger of Salzburg had published at Venice a Latin translation of the same commentary, made as early as 1841 by Father Aucher of the Mechitarist monastery of San Lazzaro from the Armenian edition of Ephraem's works published in 1836 (4 vols.). The first scholar to make this remarkable discovery widely known was Ezra Abbot in his Authorship of the Fourth Gospel (1880). Next year Westcott noticed it in the 5th ed. of his History of the N. T. Canon, and Professor Wace began his admirable series of articles in the Expositor.
Aucher and Moesinger believe this Armenian version to have been made about the 5th century, and to be a translation from the Syriac, and from the whole facts of the case the constructive argument is so strong that we may without hesitation accept this as the work of Ephraem, and conclude that the basis of this commentary is a gospel harmony, and that the Diatessaron of Tatian.
Again, the Latin harmony of Victor of Capua, as it still exists in the Codex Fuldensis (pub. by Ranke, 1868), on examination was found to be substantially identical with the fragments of Ephraem. In 1881 Zahn published his masterly monograph on Tatian's Diatessaron, containing a reconstruction of the text from the Latin, based on Moesinger's Latin version of Ephraem's Commentary, on the quotations in Aphraates, and occasional parallels with the Codex Fuldensis. He showed that Tatian's original Syriac text agreed in great part with the Curetonian Syriac, and evidently preceded the Peshito or reformed Syriac text. The fresh interest thereby aroused in the question led Ciasca of the Vatican Library to examine anew the Arabic MSS. there. The existence of one was already known, it having been partially described by Assemani, Rosenmüller, and Akerblad. In 1886 Antonios Morcos, vicar-apostolic of the Catholic Copts, forwarded to Rome a 9th-century MS., which Agostino Ciasca edited for the jubilee of Pope Leo XIII., and this was found both in contents and arrangement to correspond with the work edited by Moesinger.
Harnack thus sums up the conclusions that may be drawn from what may be considered as proved: (1) In Tatian's time there was still no recognised N. T. Canon, and the texts of the gospels were not regarded as inspired; (2) about 160 our four gospels were already in existence and authoritative, and the fourth on an equality with the three synoptics; (3) the text of the gospels in 160 was substantially the same as it is now, save that intentional changes and interpolations were made later, as the passage about the church being built upon Peter the rock.
See J. H. Hill, The Earliest Life of Christ: The Diatessaron of Tatian (translation and notes, 1893); Zahn, Forschungen zur Geschichte des neutestamentlichen Kanons (1881-91), and Geschichte des neutest. Kanons (1891); Texte und Untersuchungen zur Geschichte der Altchristlichen Literatur, by O. von Gebhardt and A. Harnack (Bd. iv. 1888); the admirable summary of the history of the Diatessaron and all the facts relating to it, by the Rev. Professor Samuel Hemphill (Dublin, 1888); and the Study by Professor J. Rendel Harris (1890); Harnack's article in the Ency. Brit., W. Möller's in Herzog's Real-Encyklopädie, and Professor Moore's in the Journal of Biblical Literature (1890); also Hilgenfeld, Die Ketzergeschichte des Urchristenthums (1884).