The Council of Ephesus, held in AD 431, was the third of the Ecumenical Councils, after Nicea in 325 and Constantinople in 381. The Council of Ephesus was primarily concerned with the doctrine of Nestorianism, though it also denounced Pelagianism and re-affirmed the Nicene Creed.
The Council of Ephesus scrutinized the ideas of the Archbishop of Constantinople, Nestorius, who had taught that Christ’s two natures, human and divine, were separate. In fact, Nestorius said that Mary ought to be referred to as “Christokos,” meaning “bearer of Christ,” not the then-traditional “Theotokos,” meaning “bearer of God.” He felt that Theotokos implied a blending of Jesus’ divine and human natures, which he believed were joined only by the will. Nestorius’ preferred term, Christokos, suggested a more complete separation of Jesus’ two natures. Historians generally agree that Nestorius’ beliefs were not dramatically different from the orthodox position. However, the debate was more than a matter of terminology in that Nestorianism conflicted with biblical concepts related to the divinity of Christ (see John 10:30).
The conflict on this point of doctrine was especially venomous between Nestorius and Cyril of Alexandria, and Cyril successfully petitioned the Pope to declare Nestorius’ views heretical. Nestorius refused to budge, however, and requested that Emperor Theodosius II call a council to settle the dispute. This request was granted, and at the Council of Ephesus Nestorius planned to denounce Cyril for heresy. Ironically, the council’s ultimate decision was exactly the opposite: it rejected Nestorianism as heretical and removed Nestorius from office.
The Council of Ephesus also rejected the concept of Pelagianism. This view held that it was possible, at least in theory, to live a morally perfect life without special aid from God. A belief in Pelagianism amounts to a rejection of the doctrine of original sin (see Romans 5:19).
In rejecting Nestorianism, the Council of Ephesus officially recognized Mary as the “Mother of God,” though at the time this term was explicitly meant to refer only to Jesus’ humanity and only appropriate in Mary’s unique circumstance. Over time, this respect for Mary would mutate, within Catholicism and Eastern Orthodoxy, into belief in her perpetual virginity, sinlessness, and cooperative work in human redemption.
Another unfortunate legacy of the Council of Ephesus is bitterness and division. The meeting itself was said to be contentious, heated, and unfriendly. The decision to condemn Nestorianism caused an immediate split in the Eastern Church, creating several splinter groups. Some of these survive today, including the Assyrian Church of the East and Chaldean Catholicism.