The Council of Ephesus - 431 AD
The third of the Ecumenical Councils was summoned by the Eastern Emperor, Theodosius II. The objective of this Council was the condemnation of the heresy of Nestorius, Patriarch of Constantinople 428-431 AD. He taught a form of Christology that dis-united the two natures of Christ in the one Divine Person. His theological heterodox Christology was refuted by prominent Orthodox theologians of the period, primarily St. Cyril of Alexandria who took Nestorius' ideas to task. Due to Nestorius' heterodox position he was deposed as the Patriarch of Constantinople.
The Council of Ephesus declared that the Virgin Mary, the Theotokos - God Bearer, was the human vessel chosen by God by which the Eternal Son would be born "in the flesh (i.e., incarnate). By this doctrinal clarification by the Church Fathers, the Council defined the uni-personality of Christ by its theological position that in the One Divine Person of Christ there are two natures - fully human and fully divine.
The Council also re-affirmed the condemnation of Pelagianism (previously condemned at the Council of Carthage, 416 AD), a heterodox teaching that rejected Original Sin and taught instead that man is solely responsible for his salvation through the exercise of good works.