The Assyrian Church of the East
It is not known exactly when Christianity first took root in upper Mesopotamia, but a Christian presence had certainly been established there by the mid-2nd century. In the 3rd century, the area was conquered by the Persians. Although this was to be a multi-ethnic church, the Assyrian people traditionally played a central role in its ecclesial life. Its geographical location caused it to become known simply as the Church of the East.
Around the year 300, the bishops were first organized into an ecclesiastical structure under the leadership of a Catholicos, the bishop of the Persian royal capital at Seleucia-Ctesiphon. He later received the additional title of Patriarch.
In the 5th century, the Church of the East gravitated towards the radical Antiochene form of christology that had been articulated by Theodore of Mopsuestia and Nestorius, and fell out of communion with the church in the Roman Empire. This was due in part to the significant influx of Nestorian Christians into Persia that took place following the condemnation of Nestorian christology by the Council of Ephesus in 431, and the expulsion of Nestorians from the Roman Empire by Emperor Zeno (474-491). In addition, the Persian Christians needed to distance themselves from the official church of the Roman Empire, with which Persia was frequently at war. In this way they were able to maintain their Christian faith while avoiding suspicions that they were collaborating with the Roman enemy.
Synods in the 5th century also decreed that celibacy should be obligatory for no one in this church, including bishops. A number of bishops and even patriarchs were married until the early 6th century, when the decision was taken to ordain only celibate monks to the episcopate. Priests, however, have always been allowed to marry, even after ordination.
The Church of the East was always a minority in largely Zoroastrian Persia, but nevertheless it flourished for many centuries, with its rich scholarly activity centered on the famous school of Nisibis. The church expanded through missionary activity into areas as far away as India, Tibet, China, and Mongolia. This continued even after the Mesopotamian homeland was conquered by the Muslim Arabs in the 7th century. The Patriarchate was moved to the new city of Baghdad after it became the capital in 766. By 1318 there were some 30 metropolitan sees and 200 suffragan dioceses. But during the invasions of Tamerlane in the late 14th century, these Christians were almost annihilated. By the 16th century, they had been reduced to a small community of Assyrians in what is now eastern Turkey. The church was then further weakened by the formation of a Catholic counterpart known as the Chaldean Catholic Church.
During World War I, the Assyrians suffered massive deportations and massacres at the hands of the Turks who suspected them of supporting the British enemy. About one third of the Assyrian population perished. Most of the survivors fled south into Iraq, hoping to be protected by the British. But in 1933, after the end of the British mandate in Iraq, a clash between Assyrians and Iraqi troops ended in another massacre and a further scattering of the community. The Iraqi authorities then stripped Assyrian Patriarch Mar Simon XXIII of his citizenship and expelled him. He went into exile in San Francisco, California, USA.