Ancient Church of the East

The Ancient Church of the East  officially the Ancient Holy Apostolic Catholic Church of the East  distinguished itself from the Assyrian Church of the East in 1964. It is one of the Assyrian churches that claim continuity with the historical Patriarchate of Seleucia-Ctesiphon – the Church of the East, one of the oldest Christian churches in Mesopotamia. The church is headquartered in Baghdad, Iraq. In 1970, Catholicos-Patriarch Mar Addai II Giwargis succeeded Mar Thoma Darmo (1968–1969).
                      

The Ancient Church of the East emerged as the result of a schism within the Assyrian Church of the East, in opposition to reforms introduced by Patriarch Mar Eshai Shimun XXIIIof the Assyrian Church of the East, including the substitution of the modern Gregorian Calendar in place of the traditional Julian Calendar (which differs from the Gregorian by a widening number of days, currently thirteen). The Ancient Church of the East seated itself in Baghdad, Iraq, headed by a separate Catholicos-Patriarch.

Following the schism, the position of Catholicos-Patriarch of the Ancient Church of the East remained vacant from 1964 until 1968, when Mar Thoma Darmo was named the church's Patriarch. Mar Thoma was elected as a rival Catholicos-Patriarch to Mar Eshai Shimun, who continued to serve as the official head of the Assyrian Church of the East headquartered in the United States. The elected Catholicos-Patriarch Mar Thoma was a native of Mesopotamia (present-day Iraq), and a former Metropolitan of the Assyrian Church of the East based in Thrissur, India. Mar Thoma served as Metropolitan from 1952 until his resignation in 1964. Upon becoming the head of the newly-formed Ancient Church of the East, he immediately relocated to Baghdad.