The Patriarch of the Church of the East was the head of the Church of the East. According to tradition, the Church of the East was founded by the apostles Thomas, Addai, Aggai, and Mari in the first century AD. At the end of the third century or beginning of the fourth century AD, Papa bar Aggai, as bishop of Seleucia-Ctesiphon, was recognised as the first supreme head of the Church of the East, according to the Chronicle of Arbela. The bishopric of Seleucia-Ctesiphon was elevated to the status of metropolitan see at the Council of Seleucia-Ctesiphon in 410 and then granted the title of catholicos at the Synod of 424. The title of patriarch was also adopted prior to the end of the fifth century.
In the Schism of 1552, the Church of the East was split into two separate lines of patriarchs following the election of Shimun VIII Yohannan Sulaqa as patriarch and his establishment of union with the Catholic Church in 1553. However, the Shimun line formally dissolved the union with the Catholic Church in 1672 and thus a third line of patriarchs in union with Rome was formed with the appointment of Joseph I as patriarch in 1681. With the end of the Josephite line in 1828 and the appointment of Yohannan VIII Hormizd as patriarch of the Chaldean Catholic Church in 1830, the Shimun line became the sole remaining line not in communion with the Catholic Church. A schism erupted again in 1968 upon the election of Thoma Darmo as patriarch of the Ancient Church of the East, whilst the Church of the East was officially renamed the Assyrian Church of the East in 1976.
