The Church of the East ( ʿĒḏtā d-Maḏenḥā) or the East Syriac Church, also called the Church of Seleucia-Ctesiphon, the Persian Church, the Assyrian Church, the Babylonian Church or the Nestorian Church, was an Eastern Christian church of the East Syriac Rite, based in Mesopotamia. It was one of three major branches of Eastern Christianity that arose from the Christological controversies of the 5th and 6th centuries, alongside the Oriental Orthodox Churches and the Chalcedonian Church. During the early modern period, a series of schisms gave rise to rival patriarchates, sometimes two, sometimes three. Since the latter half of the 20th century, three churches in Iraq claim the heritage of the Church of the East. Meanwhile, the East Syriac churches in India claim the heritage of the Church of the East in India.
The Church of the East organized itself in 410 as the national church of the Sasanian Empire through the Council of Seleucia-Ctesiphon. In 424 it declared itself independent of the state church of the Roman Empire. The Church of the East was headed by the Catholicose of the East seated in Seleucia-Ctesiphon, continuing a line that, according to its tradition, stretched back to the Apostolic Age. According to its tradition, the Church of the East was established by Thomas the Apostle in the first century. Its liturgical rite was the East Syrian rite that employs the Divine Liturgy of Saints Addai and Mari.
The Church of the East, which was part of the Great Church, shared communion with those in the Roman Empire until the Council of Ephesus condemned Nestorius in 431. Supporters of Nestorius took refuge in Sasanian Persia, where the Church refused to condemn Nestorius and became accused of Nestorianism, a heresy attributed to Nestorius. It was therefore called the Nestorian Church by all the other Eastern churches, both Chalcedonian and non-Chalcedonian, and by the Western Church. Politically the Sassanian and Roman empires were at war with each other, which forced the Church of the East to distance itself from the churches within Roman territory.