The church separated from the Assyrian Church of the East. This happened because Mar Shimun XXIII, the patriarch of Assyrian Church of the East introduced reforms which were not supported. These reforms included changes such as those of using the new Roman calendar rather than the old calendar. The people who did not support the changes thought they were opposed to the sanctity of being one of the oldest Christian churches in the world. For this reason, the followers of the Ancient Church of the East resisted the changes made to the church. Mar Thoma Darmo, a native of Mesopotamia, who was Metropolitan of the Assyrian Church of the East in India from 1952 to 1968, based at Trichur separated from the Assyrian Church of the East. He became the head of the Ancient Church of the East in October 1968 and moved to Baghdad. He died the following year. After his death, Mar Addai II was elected to replace him.