Based on various Eastern Christian traditions,[3] Addai was a Jew born in Edessa (now Şanlıurfa, Turkey).

Semi-Autonomous: Among the Eastern Orthodox faithful, Addai was a disciple of Christ[7] sent by St. Thomas the Apostle to Edessa in order to heal King Abgar V of Osroene, who had fallen ill.

[9] The story of how King Abgarus V[10][11][12] and Jesus had corresponded was first recounted in the 4th century by the church historian Eusebius of Caesarea.

[13] In the origin of the legend, Eusebius had been shown documents purporting to contain the official correspondence that passed between Abgar and Jesus, and he was well enough convinced by their authenticity to quote them extensively in his Ecclesiastical History.

(Historia Ecclesiastica, I, xiii)The story of the healing and Addai's evangelizing efforts resulted in the growing of Christian communities in southern Armenia, northern Mesopotamia and in Syria east of Antioch.