The Church of the East traces its history to the earliest planting of Christianity in the Persian Empire. The fact that the church developed outside the Roman Empire and its conciliar process partly explains its particular doctrinal tradition. It represents a third position in the three-way split in Eastern Christianity that resulted from the Christological controversies of the 5th/6th cent., alongside the Syriac (and other Oriental) Orthodox and the Chalcedonian (Melkite, Rum) Orthodox Churches. The Church of the East follows the strictly dyophysite (‘two-nature’) christology of Theodore of Mopsuestia, as a result of which it was misleadingly labelled as ‘Nestorian’ by its theological opponents.

Especially in the second half of the first millennium the Church expanded eastwards, reaching China and south India (where the E.-Syr. tradition remains prominent today; see Thomas Christians). From the 14th cent. onwards, however, the Church of the East became greatly diminished on account of plague, oppression, and periodic massacres. In the mid 16th cent. a separate patriarchal line developed, in communion with Rome (see Chaldean Catholic Church). By the 19th cent., the territory of the Church of the East was practically limited to the highlands of Hakkari in Turkey and the plain around the city of Urmia in Iran. (Some congregations in India were reclaimed by the consecration of a bp. for them in 1907.) An estimated third of its people were victims of the massacres just before, during, and in the aftermath of the First World War (see Sayfo). The remnant of the Church were forced to settle in Iraq as refugees. Following the killing of Assyrian civilians by the Iraqi army at Semele in 1933, the Catholicos Patriarch (Mar Eshai Shemʿon, then still treated as the civil head of all the Assyrians) was expelled from Iraq.

In 1968 the Church suffered a schism, the immediate cause of which was the adoption of the New Calendar mandated by Mar Shemʿon in California. The government of Iraq favored the anti-Mar Shemʿon party and facilitated the election of Mar Toma Darmo as a counter patr. Mar Darmo also made an issue of his opposition to the hereditary succession to the patriarchate. However, Mar Shemʿon had no nephews to succeed him, and in 1976, following his assassination, the new patr. Mar Denḥa IV was elected canonically. The issue of succession thus no longer divides the two Church bodies, but other historical issues (of which the calendar is only one) and animosities continue to do so. Both Churches have now (2009) agreed to hold a joint synod to try to resolve differences (in India the matter was already resolved in 1995). Both Churches have large diasporas, especially in North America (see diaspora).

1. Assyrian Church of the East: Mar Denḥa IV fixed the name of the Church as the ‘Holy Catholic Apostolic Assyrian Church of the East’. Like his predecessor he resides in the United States, although in Chicago rather than California. There are dioceses in Iraq (Baghdad, Duhok), Syria (al-Ḥasake), Lebanon (Beirut), Western Europe (Norsborg, Sweden), Western United States (Glendale, Arizona), California, Canada (Toronto), India (Trissur), and Australia (Sydney). The bp. of Duhok also has responsibility for Russia (where there is a thriving church in Moscow). Since the mid 1980s the Ch. of the East has been actively involved in Ecumenical dialogue with the Catholic Church, and in 1994 Mar Denḥa IV and Pope John Paul II issued a historic common statement on christology. In 1997 the synods of the Assyrian Church of the East and the Chaldean Catholic Church agreed to take common steps to restore union between the two Churches; this process, however, has encountered a number of obstacles and little progress has been made so far.

2. Ancient Church of the East: The Church under Mar Toma Darmo (d. 1968) and his successor Mar Addai II (1972–) adopted the name ‘Ancient Church of the East’, to difference itself from the larger ‘Assyrian’ Church (and possibly to avoid a nationalistic-sounding name in Iraq). Mar Addai is resident in Baghdad. Outside Baghdad there are dioceses of Kirkuk, Nineveh (Mosul), Syria and Lebanon (residing in al-Ḥasake), Western Europe (Mainz, Germany), United States and Canada (San Fernando), and Western United States (Modesto). The Ancient Church of the East, as well as the Assyrian Church of the East, has been present at Pro Oriente’s non-official ‘Syriac Dialogue’ since its inception in 1994.

See Fig. 35.