His teachings included a rejection of the long-used title of Theotokos, "Mother of God", for Mary, mother of Jesus, and they were considered by many to imply that he did not believe that Christ was truly God. That brought him into conflict with other prominent churchmen of the time, most notably Cyril of Alexandria, whom he accused of heresy.
Nestorius sought to defend himself at the First Council of Ephesus in 431 but instead found himself formally condemned for heresy by a majority of the bishops and was subsequently removed from his see. On his own request, he retired to his former monastery, in or near Antioch. In 435, Theodosius II sent him into exile in Upper Egypt, where he lived on until 450, strenuously defending his orthodoxy. His last major defender within the Roman Empire, Theodoret of Cyrrhus, finally agreed to anathematize him in 451 during the Council of Chalcedon.
From then on, he had no defenders within the empire, but the Church of the East never accepted his condemnation. That led later to western Christians giving the name Nestorian Church to the Church of the East where his teachings were deemed Orthodox and in line with its own teachings. Nestorius is revered as among three "Greek Teachers" of the Church (in addition to Diodorus of Tarsus and Theodore of Mopsuestia). Parts of the Church of the East's Eucharistic Service, which is known to be among the oldest in the world, is contributed to with prayers attributed to Nestorius himself.
The discovery, translation and publication of his Bazaar of Heracleides at the beginning of the 20th century have led to a reassessment of his theology in western scholarship. It is now generally agreed that his ideas were not far from those that eventually emerged as orthodox, but the orthodoxy of his formulation of the doctrine of Christ is still controversial.
He received his clerical training as a pupil of Theodore of Mopsuestia in Antioch. He was living as a priest and monk in the monastery of Euprepius near the walls, and he gained a reputation for his sermons that led to his enthronement by Theodosius II, as Patriarch of Constantinople, following the 428 death of Sisinnius I.