This third ecumenical council, an effort to attain consensus in the church through an assembly representing all of Christendom,[2] confirmed the original Nicene Creed,[3] and condemned the teachings of Nestorius, Patriarch of Constantinople, who held that the Virgin Mary may be called the Christotokos, "Christ-bearer" but not the Theotokos, "God-bearer".

Nestorius' doctrine, Nestorianism, which emphasized the distinction between Christ's human and divine natures and argued that Mary should be called Christotokos (Christ-bearer) but not Theotokos (God-bearer), had brought him into conflict with other church leaders, most notably Cyril, Patriarch of Alexandria.

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.

The proceedings were conducted in a heated atmosphere of confrontation and recriminations and created severe tensions between Cyril and Theodosius II.

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".

[7] 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.

[8] 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.

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.

This epistle drew heavily on the established Patristic Constitutions and contained the most famous article of Alexandrian Orthodoxy: "The Twelve Anathemas of Saint Cyril."

"[9] 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.

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.

Because the overland trip from Antioch to Ephesus was long and arduous, John composed his delegation of his metropolitan bishops who were restricted to bring no more than two suffragans each.

Theodosius appointed Count Candidian as the head of the imperial palace guard to represent him, to supervise the proceedings of the Council, and to keep good order in the city of Ephesus.

[21] The Palestinian delegation of 16 bishops and Metropolitan Flavian of Philippi arrived 5 days after the date that had been set for opening the council, and aligned themselves with Cyril.

[21] At this point, Cyril announced his intention to open the council; however, Candidian enjoined him from doing so on the grounds that the Roman and Antiochean delegations had not arrived yet.

[10] Cyril initially acceded to Candidian's injunction knowing that he could not legally convene a council without the official reading of the Emperor's Sacra.

[24] Various circumstances including a detour necessitated by flooding as well as sickness and death of some of the delegates seriously delayed John of Antioch and his bishops.

At this point, Cyril formally opened the council on Monday, 22 June by enthroning the Gospels in the centre of the church, as a symbol of Christ's presence among the assembled bishops.

[26] Despite three separate summons, Nestorius refused to acknowledge Cyril's authority to stand in judgment of him and considered the opening of the council before the arrival of the Antiochene contingent as a "flagrant injustice".

[10][14] The 68 bishops who opposed opening the council entered the church in protest, arriving with Count Candidian who declared that the assembly was illegal and must disperse.

[28] However, since even the bishops opposed to opening the council were now present, Cyril maneuvered Candidian by means of a ruse to read out the text of the Emperor's decree of convocation, which the assembly then acclaimed as recognition of its own legality.

Philip, as papal legate, opened the proceedings by commenting that the present question regarding Nestorius had already been decided by Pope Celestine as evidenced by his letter, which had been read to the assembled bishops in the first session.

The long letter to Celestine gave a full account of the council, and mentioned that the pope's decrees against the Pelagians had been read and confirmed.

[14] At this session, the bishops approved Canon 7 which condemned any departure from the creed established by the First Council of Nicaea, in particular an exposition by the priest Charisius.

Pope Celestine had died on 27 July but his successor, Sixtus III, gave papal confirmation to the council's actions.

[citation needed] The events created a major schism between the followers of the different versions of the council, which was only mended by difficult negotiations.

The factions that supported John of Antioch acquiesced in the condemnation of Nestorius and, after additional clarifications, accepted the decisions of Cyril's council.

[citation needed] Persia had long been home to a Christian community that had been persecuted by the Zoroastrian majority, which had accused it of Roman leanings.

The Persian Church became increasingly Nestorian in doctrine over the next decades, furthering the divide between Christianity in Persia and in the Roman Empire.