The Nestorian Schism was the split between the Byzantine church of the West and the Assyrian church of the East in the 5th century.

Nestorius was a student of Theodore of Mopsuestia at the middle School of Antioch before he became bishop of Constantinople. He was condemned by Cyril of Alexandria and the Council of Ephesus (431) for refusing to call the Virgin Mary, by the title mother of God. He would only call her mother of Christ. He was further condemned for splitting Christ into two persons, although he clearly denied that. The whole affair was complicated by the unclear arguments of Cyril, which soon provoked the Monophysite schism.

In the Syriac speaking world, and especially in the school of Edessa, Theodore was held in high esteem and the followers of his pupil Nestorius were given refuge. The Persian kings, who were at constant war with Byzantium, saw the opportunity to assure the loyalty of their Christian subjects and supported the Nestorian schism:

• They granted protection to Nestorians (462).
• They executed the pro-Byzantine Catholicos Babowai who was then replaced by the Nestorian Bishop of Nisibis Bar Sauma (484).
• They allowed the transfer of the school of Edessa to the Persian city Nisibis when the Byzantine emperor closed it for its Nestorian tendencies (489).

The writings of Nestorius were introduced at the school of Edessa-Nisibis only in about 530, a hundred years after Ephesus. The main theological authorities of the school and all the Assyrian Church have always been Theodore and his teacher Diodorus of Tarsus. Unfortunately, close to nothing of their writings has survived.

At the end of the 6th century the school went through a theological crisis when its director Henana of Adiabene tried to replace Theodore by his own doctrine, which followed Origen. Babai the Great (551-628), who was also the inofficial head of the Church at that time and revived the Assyrian monastic movement, refuted him and in the process wrote the normative Christology of the Assyrian Church, based on Theodore of Mopsuestia.

According to Babai the Great, Christ has two qnome (essences) which are unmingled and eternally united in one parsopa (personality). This, and not Nestorianism, is the teaching of the Assyrian church.