Mission to Edessa and King Abgar
After the Ascension of the Savior, Thaddeus preached the good news throughout Syria and Mesopotamia. He is most closely associated with the city of Edessa, where, according to Orthodox tradition, he converted King Abgar, the people, and the pagan priests to Christ, baptizing Abgar and all the people of the city.
He established priests in Edessa and built up the Church there. When King Abgar wished to reward him with rich gifts, Thaddeus refused them and continued on to preach in other cities, converting many pagans to the Christian Faith.
The fourth-century historian Eusebius of Caesarea first recorded the tradition of a correspondence between King Abgar and Christ, and reported that the Apostle Thomas sent Thaddeus to Edessa as a preacher and evangelist of the teaching of Christ.