He was by birth a Jew from Edessa; it was he who instructed king Avgar in the Faith and baptised him (see August 16).
According to Eusebius he is not the Thaddeus who was one of the Twelve (Mt 10:3), but was one of the Seventy.
After Christ’s Resurrection, he preached the Gospel in Mesopotamia and ended his life in martyrdom.
Troparion of the Apostle Thaddeus, Tone 4
O holy disciple and witness to God who appeared in the flesh for our sakes, thou didst bring the light of saving grace to those in darkness, thou wast revealed as an excellent physician to the people of Edessa. Wherefore shelter those who fervently seek thee.
Kontakion of the Apostle Thaddeus, Tone 4
The Church has acquired thee as a star, O Thaddeus, and is ever enlightened by thy miracles. Save those who faithfully venerate thy memory.