The third and fourth ecumenical councils held at Ephesus in 431 c.e. and at Chalcedon in 451 c.e., respectively, discussed and formulated how Christians were to speak of the relationship of Christ’s human and divine natures to one another.
Whereas the earlier ecumenical Council of Nicaea (325) and Council of Constantinople (381) had defined doctrinal perspectives on belief in the Trinity as the conviction that there was one ousia (essence) of God in three hypostases (persons), now Christology was the main concern of the councils.
At the request of Bishop Nestorius of the eastern capital Constantinople, Emperor Theodosius II called together all the major bishops of the Eastern and a few of the Western Roman Empire to meet at Pentecost 431 to resolve questions that had arisen concerning teachings advanced by Nestorius.
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Nestorius, who had been trained in the theological tradition of the school of Antioch, resisted calling Mary Theotokos (Mother of God) and preferred to speak of her as Christotokos (mother of Jesus as the one united with the Logos).
Taking advantage of travel delays among the supporters of Nestorius and hostile attitudes among delegates from Asia Minor who resented Nestorius’s claims of authority over them, Cyril took the lead at the council with a group of Egyptian monks. Following irregular proceedings, Cyril had Nestorius condemned and deposed from his position.
When other Eastern bishops, who had arrived late, met separately under Bishop John of Antioch, who was their leader, they criticized Cyril’s anathemas as fraught with Apollinarianism and Arianism and in turn condemned and deposed Cyril and Bishop Memnon of Ephesus. Joined by the Roman delegates, Cyril reconvened the council and condemned and deposed John of Antioch as well as 34 Eastern bishops.
Emperor Theodosius II approved the depositions of Nestorius, Cyril, and Memnon in early August and formally dissolved the council, yet in the confusion following the council Cyril succeeded in returning to his see of Alexandria as victor.
Nestorius, however, withdrew from the capital city to a monastery near Antioch, from where he was expelled first to Petra and then to the Great Oasis in Libya until his death after 451.
Accusations against Nestorius had focused on the claim that he divided Christ and was affirming two Christs and two Sons, a man and God, by considering the union of man and God in Christ as merely an external union.
One of the main opponents of this supposed Nestorian teaching, the influential monk Eutyches of Constantinople, promoted an extreme Monophysite (one [divine] nature) teaching that denied that Christ is homoousios (consubstantial) with humankind.
Having been condemned at a local council under the leadership of Bishop Flavian of Constantinople, Eutyches was rehabilitated by Bishop Dioscorus of Alexandria at the so-called latrocinium (Robber Council) at Ephesus in 449, a meeting remembered both for its irregularities and violence that led to the death of Flavian of Constantinople. Summoned by Emperor Marcian, the Council of Chalcedon met on October 8, 451, to resolve disputes about Monophysitism.
It rehabilitated Flavian of Constantinople and accepted as definitive the Christology formulated in Pope Leo I of Rome’s Letter to Flavian, which stated that in Christ, who is a single prosopon (person) and a single hypostasis, the complete and entire divine and human natures coexist "without mixture, without transformation, without separation, and without division". Thus Christ is homoousios both to the Father with regard to his divinity and to us with regard to his humanity.