Thaddeus of Edessa

According to Eastern Christian tradition, Thaddeus of Edessa (Syriac: ܡܪܝ ܐܕܝ, Mar Addai or Mor Aday, sometimes Latinized Addeus) was one of the seventy disciples of Jesus. He is possibly identical with Thaddaeus, one of the Twelve Apostles.
Life.
Based on various Eastern Christian traditions, Thaddaeus was a Jew born in Edessa, at the time a Syrian city, (now in Turkey). He came to Jerusalem for a festival, and heard the preachings of John the Baptist (St. John the Forerunner). After being baptized by John the Baptist in the Jordan River, he remained in Palestine. He later met and became a follower of Jesus. He was chosen to be one of the seventy disciples, whom Jesus sent in pairs to preach in the cities and places.
After Pentecost and the ascension of Jesus, Thaddeus started preaching the gospel in Mesopotamia, Syria and Persia. Thaddaeus ordained priests in Edessa, converted many to Christianity and built up the church there. He also went to Beirut to preach, and founded a church there.
The Syriac liturgy referred to as the Liturgy of Addai and Mari, originated around the year 200 AD and is used by the Assyrian Church of the East and the Chaldean Catholic Church, and Chaldean Syrian Church and Syro-Malabar Catholic Church in India founded by Thomas the Apostle.
His feast is celebrated on August 5 in the Christian calendar.
Addai and the healing of King Abgar.
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. He stayed to evangelize, and so converted Abgar—or Agbar, or in one Latin version "Acbar" — and his people including Saint Aggai and Saint Mari.
Account of the Arab King Abgarus V of the kingdom of Osroene with capital city in Edessa and Jesus had corresponded was first recounted in the 4th century by the church historian Eusebius of Caesarea 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". According to Eusebius:
The story of the healing Thaddeus' evangelizing efforts resulted in the growing of Christian communities in southern Armenia, northern Mesopotamia and in Syria east of Antioch. Thaddeus' story 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. 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 story was retold in elaborated form by Ephrem the Syrian.
Various traditions.
St. Addai also appears in the "First Apocalypse of James" and the "Second Apocalypse of James".
In Roman Catholic tradition, he and Saint Mari are considered patrons of Persian and Assyrian people.