Assyrian Church of the East/"Nestorianism": Topic PageFrom Encyclopedia of American Religious History
Given its age and location, one might think that the Assyrian Church of the East should be included among the ancient Eastern churches. The main reason for its exclusion is that the Assyrian Church's Christology, rather than rejecting the Chalcedonian view that Christ had two natures, historically affirmed that position but in a way that suggested, at least to some, there were two persons, one the man Jesus and another the divine logos within him.
This position, known as Nestorianism, after the theologian and priest Nestorius (386–451), who was believed to teach this view, was strongly condemned at the Council of Ephesus in 431. The churches in Parthia (Iran and eastern Iraq) refused to accept the decisions of that council and separated themselves from the churches of the Eastern and Western Roman Empires. This eventually led to the establishment of the Holy Apostolic Catholic Assyrian Church of the East, to give the church its formal title.