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 . In 1968 , the followers of the newly established church elected a rival catholicos-patriarch Mar Thoma Darmo while Mar Shimun XXIII continued as the official head of the Assyrian Church of the East . The elected catholicos-patriarch was a native of Mesopotamia , a former Metropolitan of the Church of the East in India from 1952 to 1968 based at Thrissur , India .