The Assyrian Church of the East is a church that traces its origins to the See of Babylon, founded by Saint Thomas the Apostle.
The Assyrian Church is the original Christian church in what was once Parthia; today Iraq and western Iran. It split from the main body of the original Christian church after the Council of Ephesus in 431, when it refused to accept that Nestorianism—the belief that Christ was two persons, one human and one divine—was a heresy. The church is considered by some Westerners to be one of the Oriental Orthodox churches, but the majority of Oriental Orthodox Churches reject any form of Nestorianism outright.
In the 15th century, the church decreed that the title of Patriarch could pass only to relatives of then-patriarch Mar Shimun IV. This upset many in the church's hierarchy, and in 1552 a rival Patriarch, Mar Yohanan Soulaqa VIII was elected. This rival Patriarch met with the Pope and entered into communion with the Roman Catholic Church.
The Assyrian Church now had two rival leaders, a hereditary patriarch in Alqosh (in modern-day northern Iraq), and a Papal-appointed patriarch in Diyarbakir (in modern-day eastern Turkey). This situation lasted until 1662 when the Patriarch in Diyarbakir, Mar Shimun XIII Denha, broke communion with Rome, and moved his seat to the village of Qochanis in the Turkish mountains. The Vatican responded by appointing a new patriarch to Diyarbakir to govern the Assyrians who stayed loyal to the Holy See. This became known as the Chaldean Catholic Church. In 1804 the hereditary line of Patriarchs in Alqosh died out, and that church's hierarchy decided to acept the authority of the Chaldean patriarchs.
Assyrians faced reprisals under the Hashemite monarchy for co-operating with the British, and most fled to the West. The Patriarch of Babylon is currently based in Chicago, and less than 100,000 of the world's 3.3 million Assyrians remain Iraq.
The Chaldean community were less numberous at the time of the British mandate, and did not play a major role in the British rule of the country. However with the exodus of Assyrians, the Chaldean Catholic Church became the largest non-Muslim group in Iraq, and many later rose to power in the Ba'ath Party government, the most prominent being Deputy Prime Minister Tariq Aziz.
The Assyrian church in India is known as the "Chaldean Syrian Church of the East".
There are also large numbers of Assyrian congregations in Iran.