Nestorian priests in a procession on Palm Sunday
, in a 7th- or 8th-century wall painting from a Nestorian church in Tang China
The Church of the East (Syriac: ܥܕܬܐ ܕܡܕܢܚܐ Ēdṯāʾ d-Maḏenḥā), also known as the Nestorian Church,[note 1] was an Eastern Christian Church originating during the late 1st century AD in Assyria, then the satrapy of Assuristan in the Parthian Empire, before spreading to other parts of Asia during the late antiquity period and throughout the middle ages. It originated as an eastern branch of Syriac Christianity, and used the East Syriac Rite in liturgy. It developed distinctive theological and ecclesiological traditions, and played a major role in the history of Christianity in Asia. Its Schism of 1552 led to a series of internal divisions among Assyrian Christians during the early modern period, and ultimately branched into the Chaldean Catholic Church and the Assyrian Church of the East. The Ancient Church of the East also traces its roots to the Church of the East.
The Church of the East was headed by the Patriarch of the East, continuing a line that, according to its tradition, stretched back to the Apostolic Age, and the christianization of the Assyrian people and other ethnic communities in western provinces of the Persian Empire. Particularly so in the historical region of Mesopotamia, including the province of Asōristān and several minor independent Mesopotamian Neo-Assyrian kingdoms of Osroene, Adiabene, Beth Garmai, Beth Nuhadra and Assur.
The Church of the East officially declared itself separate from the state church of the Roman Empire in 424–427. Liturgically, the church adhered to the East Syriac Rite. Theologically, it adopted the dyophysite doctrine of Nestorianism, which emphasises the separateness of the divine and human natures of Jesus. This doctrine and its namesake, Nestorius (c386–451), mutually rejected the Council of Ephesus in 431, leading to the Nestorian Schism and a subsequent exodus of a number of Nestorius' supporters to Sasanian Persia, which further forged the separation.
The existing Assyrian Christians in the Persian empire welcomed these refugees and gradually adopted Nestorian doctrine by the 5th century, leading the Church of Assyria to be known alternately as the Nestorian Church.
The church grew rapidly under the Sasanians, and following the Muslim conquest of Persia (633–654) it was designated as a protected dhimmi community under Islamic sharia rule. From the 6th century it expanded greatly, establishing communities in India (the Saint Thomas Christians), among the Mongols in Central Asia, and in China, which became home to a thriving community under the Tang dynasty from the 7th to the 9th century. In the 13th and 14th centuries, the church experienced a final period of expansion under the Mongol Empire, where influential Nestorian Christians sat in the Mongol court. Between the 9th and 14th centuries, at its height, the Church of the East represented the world's largest Christian church in terms of geographical extent, with dioceses stretching from its heartland in Upper Mesopotamia, from the Mediterranean Sea to as far afield as China, Mongolia, Central Asia, Anatolia, the Arabian Peninsula and India.
From its peak of geographical extent, the church experienced a rapid period of decline starting in the 14th century, due in large part to outside influences. The Mongol Empire dissolved into civil war, the Chinese Ming dynasty overthrew the Mongols (1368) and ejected Christians and other foreign influences from China, and many Mongols in Central Asia converted to Islam. The Muslim Turco-Mongol leader Timur (1336–1405) nearly eradicated the remaining Christians in Persia, Mesopotamia and the Levant; thereafter, Nestorian Christianity remained largely confined to the indigenous Assyrian communities in Upper Mesopotamia (modern Northern Iraq, southeastern Turkey, northwest Iran and, more recently, northeastern Syria) and to the Malabar Coast of India.
During the patriarchal tenure of Shemon VII Ishoyahb (1539–58), a split occurred when Shimun Sulaqa was elected as rival patriarch and entered into full communion with the Catholic Church in 1552, receiving confirmation from the pope in 1553. After the Schism of 1552, the Church of the East became divided between two branches. After several additional splits and mergers during transitional period from the middle of the 16th century up to beginning of the 19th century, both branches were finally consolidated as the Chaldean Catholic Church, one of the Eastern Catholic Churches, today with 640,828 members, and the Assyrian Church of the East, today with 170,000 members, along with the Ancient Church of the East, at 100,000.