Tatian (died c. 185) Was a second-century apologist about whom little is known apart from his writings. He was born in Assyria and was trained in Greek philosophy. While a young man he travelled extensively. Disgusted with the greed of the pagan philosophers with whom he came in contact. Repelled by the grossness and immorality of the pagans and attracted by the holiness of the Christians and the sublimity and simplicity of the Scriptures, he became a convert, probably about A.D. 150. He joined the Christian community in Rome, where he was a "hearer" of Justin. While Justin lived Tatian remained orthodox. Later (c. 172) he apostatized, became a Gnostic of the Encratite sect, and returned to the Orient.

Tatian wrote many works. Only two have survived. One of these, "Oratio ad Graecos" (Pros Hellenas), is an apology for Christianity, containing in the first part (i-xxxi) an exposition of the Christian Faith with a view to showing its superiority over Greek philosophy, and in the second part a demonstration of the high antiquity of the Christian religion. The tone of this apology is bitter and denunciatory.

The other extant work is the "Diatesseron", a harmony of the four Gospels containing in continuous narrative the principle events in the life of Our Lord. The question regarding the language in which this work was composed is still in dispute. Lightfoot, Hilgenfeld, Bardenhewer, and others contend that the original language was Syriac. Harnack, Burkitt, and others are equally positive that it was composed in Greek and translated into Syriac during the lifetime of Tatian. The "Diatesseron" or "Evangelion da Mehallete" (the Gospel of the mixed) was practically the only gospel text used in Syria during the third and fourth centuries.

Several other works written Tatian have disappeared. In his apology (xv) he mentions a work "on animals" and (xvi) one on the "nature of demons". Another work in refutation of the calumnies against the Christians (xl) was planned but perhaps never written. Eusebus says he also wrote a "Book of Problems" (Eus., "Hist. Eccl.", V, 13), dealing with the difficulties in the Scriptures, and one "On Perfection according to the Precepts of Our Saviour" (Clem. Alex., "Strom.", III, 12, 81).