Nestorius was Archbishop of Constantinople (now Istanbul) from 10 April 428 to August 431, when Emperor Theodosius II confirmed his condemnation by the Council of Ephesus on 22 June.
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 even though it has never regarded him as an authoritative teacher.
The discovery and publication of his Book of Heraclides 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. The Second Council of Constantinople of AD 553 confirmed the validity of the condemnation of Nestorius, refuting the letter of Ibas of Edessa that affirms that Nestorius was condemned without due inquiry.