Eusebius, writing early in the fourth century, says he found in the archives of Edessa a correspondence between the city's king Abgar and Jesus: the king, ill, asks him to come and heal him; the reply declines, promises to send a disciple after the ascension, and blesses those who believe without seeing. Eusebius presents this as a document he translated himself. Modern scholarship treats the letters as a composition of the third century that gave the Edessene church an apostolic charter, though the tradition of an early mission by Addai to the city rests on more than the letters alone.
A later Syriac version, the Doctrine of Addai, adds a painted portrait made by the king's messenger. Later still the portrait becomes an image imprinted on cloth by Christ himself, an image not made by hands, and the city held it as a protecting relic; a Persian siege was said to have failed because of it.
In 944 a Byzantine army obtained the cloth in exchange for prisoners and concessions and carried it to Constantinople, where it was received in a state ceremony and kept among the imperial relics until the sack of 1204, after which it disappears. Its importance for religious history is doctrinal rather than sentimental: an image believed to originate with Christ himself was the strongest available argument for the defenders of icons during the Byzantine iconoclast controversy, since it made images something God had permitted rather than something people had invented. Claims connecting it to the Shroud of Turin have been argued for decades and remain unproven.