Tatian (circa 110-180), a Christian apologist and heresiarch, was born in Assyria. He became learned in Greek literature and philosophy, and adopted the life of a travelling sophist. His conversion, which took place at Rome about 150, was due to the study of the Old Testament and the example of his Christian friends. He attached himself to Justin Martyr, and during his lifetime (probably in Greece about 153) produced the Oratio ad Graecos, an apology for the Christian religion. His Gnostic and ascetic tendencies, which came to a head about 172, occasioned his withdrawal to Mesopotamia, where he opened a school, probably at Edessa. He afterwards became the leader of the Encratites. His Diatessaron, composed of materials furnished by the four evangelists, throws nmch light on the history of the canon.