early bishop of Constantinople whose views on the nature and person of Christ led to the calling of the Council of Ephesus in 431 and to Nestorianism, one of the major Christian heresies. A few small Nestorian churches still exist. (See also Nestorian.)

Nestorius was born of Persian parents. He studied at Antioch (now in Turkey), probably as the pupil of Theodore, bishop of Mopsuestia. He became a monk at the nearby Monastery of St. Euprepius and, after being ordained a priest, acquired a great reputation for asceticism, orthodoxy, and eloquence. Owing to this reputation, Nestorius was nominated by the Eastern Roman emperor Theodosius II to become bishop of Constantinople in 428. His debut was a stormy one, however, for he immediately set to work extirpating heretics of every sort, showing leniency only to Pelagians.

A crisis developed when Nestorius' domestic chaplain, Anastasius, on Nov. 22, 428, preached a sermon in which he objected to the title Theotokos (“God-Bearer”) as applied to Mary. Many were scandalized, for the term had long been in use. Nestorius, who had already expressed doubts on the subject, supported Anastasius and, on Christmas Day, began a series of addresses arguing that Mary was not Theotokos. Nestorius considered that, unless carefully qualified, the term Theotokos as applied to Mary compromised Christ's full humanity. To many people it seemed that Nestorius himself was denying the divinity of Christ and regarding him as a mere man who had been adopted by God as his son. In the resulting controversy, Nestorius' opponents found an ally in Cyril (Cyril of Alexandria, Saint), bishop of Alexandria, whose intervention set in motion a series of events that finally led to Nestorius' downfall.

Cyril's motive was twofold. He sincerely believed that Nestorius was undermining the purity of the faith; Nestorius' refusal to style Mary as Mother of God seemed to Cyril to deny the unity of the God-man. As bishop of Alexandria, Cyril also was eager to belittle the rival see of Constantinople, as his immediate predecessor, Theophilus, had done in the case of John Chrysostom. Theological and political considerations were thus combined.

Both sides appealed to Pope Celestine I (Celestine I, Saint), who had already been offended by Nestorius' tactlessness. In August 430 Celestine held a church council in Rome which decided that correct Christology required the use of the term Theotokos and requested Nestorius to disown his errors. When Cyril, who had been authorized to execute the sentence upon Nestorius, produced a string of provocative anathemas for him to subscribe to or face excommunication, Nestorius and his allies took alarm, and he persuaded the emperor Theodosius to convene a general church council. It was Nestorius' hope that the council would result in the condemnation of Cyril. When the council met at Ephesus (Ephesus, councils of) in 431, however, Nestorius found himself hopelessly outmaneuvered by Cyril. (See Ephesus, councils of.)

Nestorius' teaching was condemned and he himself was deposed from his see. Theodosius was induced to ratify these decisions, and Nestorius was relegated to his former monastery near Antioch. After languishing there in exile for four years (431–435), he was transferred to the Great Oasis (now the Oasis of Khārijah) in the Libyan Desert about 436 and was later removed to Panopolis in Upper Egypt. During his exile, he wrote the Book of Heraclides of Damascus, which he intended as a defense of his teaching and a history of his life. The sole treatise from his pen to have survived, it was discovered in 1895 in a Syriac translation. Nestorius died in Panopolis about 451, protesting his orthodoxy.

Nestorius is regarded as one of the principal heretics in Christology, and the heresy traditionally linked with his name, Nestorianism, was formally condemned at the church councils of Ephesus (431) and Chalcedon (451). Nestorianism, as it was understood at the time, so insisted upon the full humanity of Christ's human nature that it was believed to divide him into two persons, one human and the other divine. Whereas orthodox Christology holds that Christ has two natures, divine and human, ineffably united in one person, or hypostasis, Nestorianism so stresses their independence as to suggest that they are in effect two persons, or hypostases, loosely joined by a moral union. Nestorianism envisages the divine Word as having associated with itself at the incarnation a complete, independently existing man. From the orthodox point of view, Nestorianism therefore denies the reality of the incarnation and represents Christ as a God-inspired man rather than as God-made-man. Since the 5th century all the principal branches of the Christian church have united in condemning Nestorianism and have affirmed that Christ is a single person, at once wholly human and wholly divine. Even the so-called Nestorian church is not Nestorian in the strict sense, though it venerates Nestorius and refuses to accept the title Theotokos.

It is questionable, however, whether Nestorius himself ever taught, or intended to teach, the heresy named for him. Indeed, he repudiated such views, and Cyril's attacks on him were based on a misunderstanding. The fact is that Nestorius repeatedly affirmed the perfect unity of the incarnate Christ, and he repudiated any suggestion of there being two persons existing side by side in his being. Nestorius can be better understood as the victim of his own intolerant personality and crudely provocative rhetoric, and as having been the loser in one of the rivalries between great episcopal sees that were a feature of the time.

What Nestorius actually taught was a prosopic union. The Greek term prosōpon means the external, undivided presentation, or manifestation, of an individual that can be extended by means of other things—e.g., a painter includes his brush within his own prosōpon. So the Son of God used manhood for his self-manifestation, and manhood was, therefore, included in his prosōpon, so that he was a single object of presentation.

Nestorianism was crushed within the Roman Empire but survived outside its frontiers. The Christian church in Persia adopted it, largely to obtain the protection of its rulers by assuring them that its religion was not that of their enemies the Romans. Nestorianism has continued until today, though its adherents are few, with groups being found in Iraq, India, Iran, Syria, and North and South America.