Around the year 170, Tatian from "the country of the Assyrians" (most probably Adiabene), after his return from Rome where he has been won over to Christianity by Justin Martyr, created his gospel concord, Diatessaron (to dia tessarwn euaggelion). This very quickly appeared in Syriac. Its original language is thought to have been Greek, as is probable, but not certain, from the Dura-Europos fragment dating from the middle of the 3rd Century, or certainly before 256-7 when Shapur I conquered the town. Such a work presupposes a Christian public of some dimensions, and it must have filled a need. At the end of the 2nd century, Bishop Aberkios of Hieropolis found fellow Christians in the Nisibis area: on the basis of the references in the Vita of Aberkios (4th century) to his meeting with Bardaisan (BarcasanhV) this is probably historically correct. Bardaisan himself was a Christian; at least, his teaching was strongly influenced by Christian ideas, although his concept of Christianity was stamped as heretical in later heresiology, particularly Afrem. In the "Book of the Laws of Countries," attributed to him and edited by the disciple Philippus at the beginning of the 3rd century, he refers to Christian brothers and sisters on Iranian territory. There is also the report of about the same period in the "Chronicle of Edessa" concerning the destruction of part of the Christian Church in the city in the catastrophic flood in the year 201. The reference in Eusebius (Ecclesiastical History, v.23.4) to the active participation of the Osrhoene community in the controversy about the Easter festival under Bishop Victor of ROme (189-99) is, however, although historically probable, uncertain, as this actual passage is not found in Rufin's Latin translation.

From these sources it is clearly apparent that, in view of the time necessary to establish even a fairly small community, Christian communities existed in eastern areas from the beginning of the 2nd century, and that during the century these communities consolidated themselves by some form of organization. But one must not be led to belief that Christianization was particularly radical, or that the structure of the Church was particularly extensive. For example, names in an inscription from the Edessa area indicate that a pagan cult was still extensive in the 2nd and 3rd centuries.

-The Cambridge History of Iran, vol. 3(2): The Seleucid, Parthian, and Sasanian Periods, ed. Ehsan Yarshater (NY: Cambridge UP, 1983), 927-928.