The Christological question which formed the background to the Nestorian controversy: "How are divinity and humanity joined together and related to each other in Jesus Christ?"
The Western Church affirmed Tertullian's formula: in Christ, there are two natures united in one person.
The Eastern Church had two schools of thought: the Antiochene and the Alexandrian.
The Antiochene school was influenced by Aristotle and adhered to an historical exegesis (i.e. concentrating on what the Bible actually said), affirming that Jesus was fully human, that the Godhead dwelt in him, but did not eclipse his humanity.
The Alexandrine school was influenced by Plato and followed an allegorical tradition (i.e. tending to attach several layers of meaning to every text), affirming that Jesus' divinity must take precedence, even if at the expense of his humanity.
The Antiochenes spoke of two natures in Christ, so they came to be known as Dyophysites (from the Greek duo physis, "two natures"), whereas the Alexandrians insisted upon one nature, at once divine and human, so they came to be known as Monophysites (from mono physis, "one nature").
In order to preserve the emphasis on oneness, it was difficult for the Alexandrians not to weaken either the deity or the humanity of Christ; in the view of Antioch, they tended to do the latter.
Antioch considered that Alexandria devalued the humanity of Jesus, whereas Alexandria looked upon Antioch as overemphasizing his humanity.

2) Political Background

Prior to the fourth century, Alexandria had been second only to Rome as the greatest patriarchate.
The Council of Constantinople in 381 had declared that Rome and Constantinople were equal, thus demoting Alexandria from its former position.
Since Constantinople held a higher position than Antioch or Alexandria, the bishops of both competed for the honor of being the Patriarch of Constantinople.
Since the Antiochenes were more successful than the Alexandrines in occupying the Patriarchate, the latter regarded both Antioch and Constantinople somewhat as enemies.
There was a history of animosity between the Patriarch of Alexandria and the Patriarch of Constantinople.
John Chrysostom, a presbyter in Constantinople, became Patriarch in 398; he was a fearless and dedicated reformer, as well as a former pupil of Diodore of Tarsus (d. 394) and fellow student with Theodore of Mopsuestia (c. 350-428).
Theophilus, Patriarch of Alexandria, an ambitious prelate, was nominated to be John's consecrator.
Theophilus lived as a great magnate, while John was an ascetic whose main concern was social justice and charity to the poor.
John's campaign to evangelize the city resulted in opposition from clergy and others who resented his pure life and uncompromising zeal.
His greatest opponent was Theophilus, who was jealous of the popularity of his rival and of the priority of honour enjoyed by Constantinople.
Theophilus assembled a synod of bishops (most from Egypt) in Constantinople in 403 and summoned John before them, but he did not appear, so they condemned him in his absence on various false charges.
John protested his innocence, but surrendered to the Imperial bodyguard and left Constantinople

3) Nestorius and His Theological Influences

Nestorius, a Syrian monk from Antioch, was elected Patriarch of Constantinople in 428, possibly because he was a popular preacher.
Prior to his election, he had been a relatively obscure priest.
Upon election to his new position, he embarked on a campaign of persecution against Arians and other heretics.
He had been influenced by the Christology of Diodore of Tarsus and Theodore of Mopsuestia, under whom he probably studied.
Diodore presented Christ as having two natures, human and divine; the divine Logos indwelt the human body of Jesus in the womb of Mary, so that the human Jesus was the subject of Christ's suffering, thus protecting the full divinity of the Logos from any hint of diminishment.
Theodore, the father of Antiochene theology, taught two clearly defined natures of Christ: the assumed Man, perfect and complete in his humanity, and the Logos, consubstantial with the Father, perfect and complete in his divinity, the two natures (physis) being united by God in one person (prosopon).
Theodore maintained that the unity of human and divine in Jesus did not produce a "mixture" of two persons, but an equality in which each was left whole and intact.
Diodore and Theodore were considered orthodox during their lifetime, but came under suspicion during the Christological controversies of the fifth century.
The Syriac Fathers (including Diodore, Theodore, and Nestorius) used the Syriac word kyana to describe the human and divine natures of Christ; in an abstract, universal sense, this term embraces all the elements of the members of a certain species, but it can also have a real, concrete and individual sense, called qnoma, which is not the person, but the concretized kyana, the real, existing nature.
The Greek word prosopon (person) occurs as a loan word parsopa in Syriac; thus, the Syriac Christological formula was "Two real kyana united in a single parsopa, in sublime and indefectable union without confusion or change."
Whereas Antioch taught that Christ had two natures (dyophysitism), Alexandria interpreted their position as teaching that he had two persons (dyhypostatism).
Whereas the Syriac Fathers were willing to leave the union of Christ's humanity and divinity in the realm of mystery, the Alexandrians sought a clear-cut doctrine that would guard the church against heresy.

4) The Teaching of Nestorius

At the time, Theotokos ("bearer/mother of God") was a popular term in the Western Church (including Constantinople) used to refer to the Virgin Mary, but it was not used in Antioch.
Nestorius maintained that Mary should be called Christotokos ("bearer/mother of Christ"), not Theotokos, since he considered the former to more accurately represent Mary's relationship to Jesus.
Nestorius promoted a form of dyophysitism, speaking of two natures in Christ (one divine and one human), but he was not clear in his use of theological terms.
Nestorius spoke of Christ as "true God by nature and true man by nature... The person [parsopa] is one... There are not two Gods the Words, or two Sons, or two Only-begottens, but one."
Alexandria understand him to mean that the second person of the Trinity was actually two persons: the man Jesus who was born, suffered and died and the divine Logos, eternal and unbegotten.
Part of the problem lay in his use of the Greek word prosopon (Syriac parsopa) for "person"; this word was weaker in meaning than hypostasis, the word used by his opponents.
At no time did he deny Christ's deity; he merely insisted that it be clearly distinguished from his humanity.

5) Cyril of Alexandria

Cyril, Patriarch of Alexandria (412-444) and the nephew of Theophilus, opposed Nestorius (he was a more able politician and theologian).
His zeal for Orthodoxy was not accompanied by charity to his rivals and from the first his rule was marked by the acts of violence of his fanatical followers.
Cyril was driven by the ambition to assert Alexandria's primacy over Antioch and Constantinople.
Cyril maintained that in Christ the divine and the human nature were both complete and that the latter included the rational element; the unity in Christ was through the Logos who became incarnate in Christ and took on the general characteristics of man.
Cyril saw Christ's humanity as that of humanity in general, not that of an individual man; salvation was accomplished by the personal Logos who assumed impersonal human nature, thus uniting it with the divine nature.
Cyril championed the use of Theotokos and accused Nestorius of teaching that Christ had been a "mere man."
Cyril's critics had been complaining of him to Emperor Theodosius II and to Nestorius, so Cyril was eager to shift attention away from himself and onto Nestorius.
Cyril gained the support of the Western and Eastern Roman Emperors and the Pope.

6) The Council of Ephesus

Emperor Theodosius II convened an ecumenical council at Ephesus in 431.
A synod at Rome in 430 had ordered Nestorius either to recant or to be excommunicated.
At another synod in Alexandria in 430, Cyril issued 12 anathemas against Nestorius and various propositions taught in Antioch; apart from his reluctance to use Theotokos, Nestorius was not guilty of any of the accusations brought against him.
Nestorius and others of the Antiochene school counter-attacked, accusing Cyril of heresy.
Nestorius' supporters, the Oriental bishops led by John, Patriarch of Antioch, were delayed on their way to the council; Nestorius himself refused to attend the council until John's party had arrived.
Cyril summoned his followers, opened the council, and excommunicated Nestorius before John's arrival.
When John and his party reached Ephesus and heard of this, they in turn excommunicated Cyril and his ally Memnon, Archbishop of Ephesus.
When Celestine, the Bishop of Rome (i.e. the Pope) arrived, the reconvened council excommunicated John and his party.
Both sides appealed to the Emperor, who confirmed the excommunications of Cyril, Memnon and Nestorius.
Nestorius accepted the verdict and spent the rest of his life in exile in Upper Egypt, dying in obscurity.
Cyril bribed his way back to power, returning to Egypt, where he continued on as Patriarch, dying amidst the trappings of ecclesiastical splendour.
In 433, a peace by compromise was concluded between Cyril and John; Cyril retained his patriarchate, but withdrew his anathemas against Antioch, while the Oriental bishops accepted the use of Theotokos and sacrificed Nestorius by agreeing to his excommunication.
After the deaths of John in 442 and Cyril in 444, the compromise collapsed.

7) The Council of Chalcedon

The Council of Chalcedon (451) produced a "Definition of Faith" about Christ that was essentially Dyophysite in nature, thus alienating the Monophysite churches (the Syrian, Coptic, Armenian and Ethiopian Orthodox Churches), who separated from the Western Church after this council.
Chalcedon defined Christ as "one person in two natures [rather than the Monophysite form "out of two natures"], human and divine."
Chalcedon was unable to define the relationship of the two natures to each other, but confessed that the two are not destroyed by the union in the one person, but are preserved "without confusion, without change, without division, without separation."
From his exile, Nestorius condemned the heresy falsely attributed to him, that the human Jesus and the divine Christ were two different persons, and asserted that Jesus Christ was one Lord, indivisible in his person (prosopon), but containing two natures (ousiai), the divine and the human.

8) Summary

Nestorius spoke of Christ as one person (prosopon) in two natures (physis), human and divine.
The Monophysites spoke of him as one person (hypostasis) and one nature (physis), both God and man.
Chalcedon referred to Christ as one person (hypostasis) in two natures (physis), in essence a compromise between the Nestorian and Monophysite positions.
The Nestorian bishops, in a statement drawn up in 612, stated: "There is a wonderful connection and indissoluble union between [Christ's] human nature, which was assumed, and God the Word who assumed it, a union existing from the first moment of conception. This teaches us to recognize only one Person (parsopa), our Saviour Jesus Christ, Son of God, begotten in the nature of his Godhead by the Father before all ages, without beginning, and born finally in the nature of his Manhood of the holy Virgin, the daughter of David."

Sources:

1. Ferguson, Everett, Michael P. McHugh and Frederick W. Norris. The Encyclopedia of Early Christianity. New York: Garland Publ., 1990.

2. Gonzalez, Justo L. The Story of Christianity, Vol. I: The Early Church to the Dawn of the Reformation. New York: Harper Collins, 1984.

3. Latourette, Kenneth Scott. A History of Christianity. New York: Harper & Brothers, 1953.

4. Maloney, George. "Dialogues Between the Assyrian Church of the East and the Church of Rome," in Diakonia, Vol. 29, No. 3 (1996), 204-214.

5. Moffett, Samuel Hugh. A History of Christianity in Asia. San Francisco: HarperSanFrancisco, 1992.

6. Zernov, Nicholas. Eastern Christendom. New York: G.P. Putnam's Sons, 1961.

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