VIb. The Church of the East, the East Syrians or "Nestorians" (Excursus)

Of the Apostles it is Thomas, along with Thaddeus of Edessa and Mari, who is associated with the spread of the gospel in Mesopotamia; there were established Christian communities here by the mid-third century. The lingua franca of these areas was Syriac, closely related to the Aramaic spoken by the Lord. These churches initially came under the patriarchal see of Antioch in Syria. The political power in control of this area was first the Parthian Empire, and then, from 224, the Sasanid Empire.

From 311 Christianity was first tolerated and then officially adopted in the Roman Empire. This meant that Christians in the Sasanid Empire become politically suspect, and there were persecutions of Christians, particularly under rulers Shapur II (339-379) and Khosrau I (ending 545). 

In the fifth century, both for the above political reasons, and also in connection with the Nestorian controversy, the Christian church in the Persian empire became autonomous. In 410 the Catholicos of Ctesiphon was declared Grand Metropolitan (later Patriarch), and in 424 independent of the "western fathers". After the condemnation of Nestorius in 431, in 484 the self-styled "Church of the East", i.e. the Christian church in the Sasanid Empire, affirmed the theology of Theodore of Mopsuestia, which emphasised (some would say over-emphasised) the two distinct natures of Christ (the Nestorian tendency). In 489 the theological school at Edessa was moved to Nisibis within the borders of the Persian Empire. The theology of the Church of the East was further articulated by Babai the Great (551-628) in his work, Book of Union, which speaks of Christ's "two qnome (natures)". 

Following the Muslim conquest of Persia in 644, the Christians of the Seleucia Catholicate-Patriarchate made a major contribution to the Umayyad and Abbasid Caliphates, translating works into Arabic, and also in the fields of philosophy, science and medicine. 

Already by the sixth century the Church of the East had expanded with missionary work into Central Asia, India and among the Mongols. By 635 the Church of the East had reached China, as commemorated by the Nestorian stele. The scrolls found at Turfan (modern-day Xinjiang province, China, dated to the eighth century) include many Christian texts in Syriac, Sogdian and Turkic (Uighur). 

A major figure in the Church of the East was Patriarch Timothy I (780-823) under whose ministry missionary work progressed further. Incidentally, Timothy I's Dialogue with Al-Mahdi demonstrates both a powerful Christian apologetic vis-à-vis Islam, and also the amicable relations which existed between the Muslim Abbasid rulers and their Christian subjects. The period from 800-1200 represents the apex of the Church of the East, embracing the Kara Khitai (in modern-day Kyrgyzstan), India (Kerala and Patna), and the Kerait, Naimans and Merkits (north of China). Navekath in modern-day Kyrgyzstan was a major episcopal see, and in this area there are remains of churches with open-air naves, arguably an architectural form later used by Muslim mosques. There are even possible Christian sites in Korea (Geongju) and Japan (Koryungi Temple). 

In 1368 the Mongol dynasty in China was overthrown and the Mongols, along with the Christians, thrown out. This was followed by the rule of Tamerlane (1370-1405), who almost completely eradicated the church. Only seven bishoprics survived.