Saint Addai was the evangelizer of Edessa and is believed to be one of the Seventy Apostles of Christ, known to Eusebius of Caesarea as the Apostle Thaddeus. He is purported by Eusebius to have visited King Abgar of Edessa, healing him and preaching to him. The Syriac account of this story is titled the Teaching of Addai, and there are questions about its historicity. The story possibly dates instead from the 3rd century and may have been used in polemics against the Manichaean "apostle" also named Addai.
The Syriac liturgy referred to as the Liturgy of Addai and Mari, which originated around the year 200 A.D. and is used by the Assyrian Church of the East, claims a connection to this saint.