Thaddeus, called Addai in the Syriac tradition, was a Jew of Edessa by origin and came to Jerusalem for a feast. He heard the preaching of John the Forerunner, was baptized, and afterward followed Christ, and was numbered among the seventy whom the Lord sent out ahead of Him two by two.

The story attached to his name is the story of Abgar the Fifth, called Ukkama, the Black, king of Osrhoene, whose capital was Edessa on the far eastern edge of the Roman world. Eusebius, writing early in the fourth century, says he found the documents in the public record office of Edessa, in Syriac, and translated them: a letter from the king to Jesus, and a reply.

The king, in the letter as Eusebius gives it, has heard of the healings, believes the healer to be either God or the Son of God, invites Him to come to Edessa, and offers Him the shelter of a small city out of reach of the Jerusalem authorities. The reply blesses him for believing without having seen and says that the mission must be finished first, and that afterward one of the disciples will be sent to heal him.

After the Ascension, Eusebius continues, Thomas sent Thaddeus, one of the seventy, to Edessa. He lodged with a man named Tobias and began to heal, and the report of it reached the king, who understood at once who had sent him. Abgar was healed. The city heard the preaching, and Edessa became, on this account, the first state whose ruler professed the Christian faith, generations before Armenia and long before Rome.

The Syriac Doctrine of Addai, which is later and much fuller, adds the elements that the Eastern tradition has kept: that a court painter was sent to make a portrait of Christ and could not, and that Christ pressed a cloth to His face and sent the image instead. That image, the Holy Napkin, the Mandylion, the Image Not Made by Hands, is the subject of the feast this app keeps on the sixteenth of August, the translation of the image from Edessa to Constantinople in the year nine hundred and forty four.

Historians treat the Abgar correspondence as a document of the church of Edessa rather than a transcript, and generally date the conversion of the royal house to Abgar the Eighth, around the year two hundred, rather than to the first century. The Church has never required a reader to hold one view of that question. What is not in dispute is that Edessa was Christian very early, that it held Thaddeus for its apostle from the beginning, and that the Syriac-speaking Christian world traces its foundation to him.

The tradition sends him on from Edessa into Mesopotamia and Syria and finally to Beirut in Phoenicia, where he preached and where he reposed, according to the Menaion, in the year forty four. Some accounts have him martyred, others have him die in peace; the Church commemorates him as an apostle rather than settling the manner of the death.

His memory is kept on the twenty first of August, and among the Seventy on the fourth of January, the Synaxis of the Seventy Apostles. In the Syriac churches he is Mar Addai, and the ancient liturgy of the East, still served, carries his name.