Mar Aggai was one of the earlier leaders of the Church of the East. According to tradition, he was one of the seventy apostles who, when Jesus divided the world into various mission regions, was given the area of Mesopotamia as far as the border of India. He was appointed by Mar Addai to be his successor shortly before Addai died. Mar Aggai, like Addai before him, sent missionaries to the east. He is said to have been murdered in church by one of the sons of King Abgar V of Edessa. Shortly before his death, Aggai is reported to have appointed Palut as his successor.

Among the Eastern Orthodox faithful, Saint Addai was a disciple of Christ sent by St. Thomas the Apostle to Edessa in order to heal King Abgar V of Osroene, who had fallen ill. St. Addai stayed to evangelize, and converted Abgar—or Agbar, or in one Latin version "Acbar" — and his people including Saint Aggai and Saint Mari. He is known as one of the great apostles to Syria and Persia. He is considered to have been one of the early Catholicoses of the East, following Saint Thomas the Apostle. He and Saint Mari are credited with the Divine Liturgy of Addai and Mari. St. Addai is also known as Addeus— or Thaddeus which is a doublet for St. Jude the Apostle.

The story of St. Addai, the apostle of Edessa, accounts for the growing Christian communities in northern Mesopotamia and in Syria east of Antioch. The identity of the specific Agbar/Abgar is open to interpretation: see Abgar. The fully developed legend of Addai is embodied in the Syriac document, Doctrine of Addai, which recounts the role of Addai and makes him one of the 72 Apostles sent out to spread the Christian faith. The legendary tale of how King Abgarus V of Edessa and Jesus had corresponded was first recounted in the 4th century by the church historian Eusebius of Caesarea and it was retold in elaborated form by Ephrem the Syrian. In the origin of the legend, Eusebius had been shown documents purporting to contain the official correspondence that passed between Abgar and Jesus, and he was well enough convinced by their authenticity to quote them extensively in his Ecclesiastical History. By the time the legend had returned to Syria, the purported site of the miraculous image, it had been embroidered into a tissue of miraculous happenings: the Doctrine of Addai is full of miracles, and antisemitism in the garbled story of "Protonike" consort of Claudius, searching for the Cross, and Golgotha and the Holy Sepuchre, all of them in possession of the Jews.

St. Addai appears in unorthodox material as well, in two previously unknown apocalypses attributed to James the Just found at Nag Hammadi in 1945.

Death

Saint Addai is believed to have died in northern Iraq, in a Chaldean village named Yarda. the village is now deserted, due to the Saddam regime that forced most villages of northern Iraq to move to the major cities, because of the danger that lies within that area.