The Council of Ephesus was a council of Christian bishops convened in Ephesus (near present-day Selçuk in Turkey) in AD 431 by the Roman emperor Theodosius II. This third ecumenical council, an effort to attain consensus in the church through an assembly representing all of Christendom, confirmed the original Nicene Creed, and condemned the teachings of Nestorius, Patriarch of Constantinople, who preferred that the Virgin Mary be called Christotokos, "Christ-bearer" over Theotokos, "God-bearer"; in contrast to Cyril of Alexandria who deemed that Theotokos is enough on its own. It met from 22 June to 31 July 431 at the Church of Mary in Ephesus in Anatolia.
Nestorius' doctrine, Nestorianism, which emphasized the distinction between Christ's human and divine natures and argued that Mary should preferably be called Christotokos (Christ-bearer) over Theotokos (God-bearer), had brought him into conflict with other church leaders, most notably Cyril, Patriarch of Alexandria. Nestorius himself had requested the Emperor to convene the council, hoping that it would prove his orthodoxy; the council in fact condemned his teachings as heresy. The council declared Mary as Theotokos (Mother of God).
Nestorius' dispute with Cyril had led the latter to seek validation from Pope Celestine I, who offered his support for Cyril to request that Nestorius recant his position or face excommunication. Nestorius pleaded with the Eastern Roman Emperor Theodosius II to call a council in which all grievances could be aired, hoping that he would be vindicated and Cyril condemned.
Approximately 250 bishops were present. The proceedings were conducted in a heated atmosphere of confrontation and recriminations and created severe tensions between Cyril and Theodosius II. Nestorius was decisively outplayed by Cyril and removed from his see, and his teachings were officially anathematized. This precipitated the Nestorian Schism, by which churches supportive of Nestorius, especially in the Persian Empire of the Sassanids, were severed from the rest of Christendom and became known as Nestorian Christianity, or the Church of the East, whose present-day representatives are the Assyrian Church of the East, the Ancient Church of the East, the Chaldean Syrian Church, and the Chaldean Catholic Church (which restored communion with Rome).
John McGuckin cites the "innate rivalry" between Alexandria and Constantinople as an important factor in the controversy between Cyril of Alexandria and Nestorius. However, he emphasizes that, as much as political competition contributed to an "overall climate of dissent", the controversy cannot be reduced merely to the level of "personality clashes" or "political antagonisms". According to McGuckin, Cyril viewed the "elevated intellectual argument about Christology" as ultimately one and the same as the "validity and security of the simple Christian life".
Even within Constantinople, some supported the Roman-Alexandrian, and others supported the Nestorian factions. For example, Pulcheria supported the Roman-Alexandrian popes while the emperor and his wife supported Nestorius.
Contention over Nestorius' teachings, which he developed during his studies at the School of Antioch, largely revolved around his rejection of the long-used title Theotokos ("Carrier of God") for the Virgin Mary. Shortly after his arrival in Constantinople, Nestorius became involved in the disputes of two theological factions, which differed in their Christology.
McGuckin ascribes Nestorius' importance to his being the representative of the Antiochene (an adjectival form of the word 'Antioch') tradition and characterizes him as a "consistent, if none too clear, exponent of the longstanding Antiochene dogmatic tradition". Nestorius was greatly surprised that what he had always taught in Antioch without any controversy whatsoever should prove to be so objectionable to the Christians of Constantinople. Nestorius emphasized the dual natures of Christ, trying to find a middle ground between those who emphasized the fact that in Christ God had been born as a man, and who insisted on calling the Virgin Mary Theotokos (Greek: Θεοτόκος, "God-bearer"), and those that rejected that title because God as an eternal being could not have been born. Nestorius suggested the title Christotokos (Χριστοτόκος, "Christ-bearer"), but this proposal did not gain acceptance on either side.
Nestorius tried to answer a question: "How can Jesus Christ be God and man at the same time?" He taught that Mary, the mother of Jesus gave birth to the incarnate Christ, not the divine Logos, which referred to the Logos in its divine nature, according to Antiochene usage, who existed before Mary and indeed before time itself. Consequently, Nestorius argued that the Virgin Mary should be called Christotokos, Greek for "Carrier of Christ", and not only Theotokos, Greek for "Carrier of God".
According to McGuckin, several mid-twentieth-century accounts have tended to "romanticise" Nestorius; in opposition to this view, he asserts that Nestorius was no less dogmatic and uncompromising than Cyril, and that he was clearly just as prepared to use his political and canonical powers as Cyril or any of the other hierarchs of the period.
Nestorius's opponents charged him with detaching Christ's divinity and humanity into two persons existing in one body, thereby denying the reality of the Incarnation. Eusebius, a layman who later became the bishop of the neighbouring Dorylaeum, was the first to accuse Nestorius of heresy, but his most forceful opponent was Patriarch Cyril of Alexandria. Cyril argued that Nestorius's two-natures formula necessarily leads to two Sons.
Cyril appealed to Pope Celestine I of Rome, charging Nestorius with heresy. The Pope agreed and gave Cyril his authority to serve a notice to Nestorius to recant his views within ten days or else be excommunicated. Before acting on the Pope's commission, Cyril convened a synod of Egyptian bishops, which condemned Nestorius as well. Cyril then sent four suffragan bishops to deliver both the Pope's commission as well as the synodal letter of the Egyptian bishops. Cyril sent a letter to Nestorius known as the "Third Epistle of Saint Cyril to Nestorius". This epistle drew heavily on the established Patristic Constitutions and contained the most famous article of Alexandrian Orthodoxy: "The Twelve Anathemas of Saint Cyril". In these anathemas, Cyril excommunicated anyone who followed the teachings of Nestorius. For example, "Anyone who dares to deny the Holy Virgin the title Theotokos is Anathema!" Nestorius, however, still would not repent. McGuckin points out that other representatives of the Antiochene tradition such as John of Antioch, Theodoret and Andrew of Samosata were able to recognize "the point of the argument for Christ's integrity" and concede the "ill-advised nature of Nestorius' immoveability". Concerned at the potential for a negative result at a council, they urged Nestorius to yield and accept the use of the title Theotokos when referring to the Virgin Mary.
For example, John of Antioch wrote to Nestorius urging him to submit to the Pope's judgment and cease stirring up controversy over a word that he disliked (Theotokos) but which could be interpreted as having an orthodox meaning especially in light of the fact that many saints and doctors of the church had sanctioned the word by using it themselves. John wrote to Nestorius, "Don't lose your head. Ten days! It will not take you twenty-four hours to give the needed answer.... Ask advice of men you can trust. Ask them to tell you the facts, not just what they think will please you.... You have the whole of the East against you, as well as Egypt." Despite this advice from his colleagues, Nestorius persisted in maintaining the rightness of his position.
On 19 November, Nestorius, anticipating the ultimatum decision which was about to be delivered, convinced Emperor Theodosius II to summon a general council through which Nestorius hoped to convict Cyril of heresy and thereby vindicate his own teachings. Theodosius issued a Sacra calling for the metropolitan bishops to assemble in the city of Ephesus, which was a special seat for the veneration of Mary, where the theotokos formula was popular. Each bishop was to bring only his more eminent suffragans. The date set by the Emperor for the opening of the council was Pentecost (7 June) 431.