Third Ecumenical Council
The Third Ecumenical Council was held in Ephesus, Asia Minor in 431 under Emperor Theodosius II, grandson of Theodosius the Great. Approximately 200 Bishops were present, though procedings began in haste before the arrival of the bishops from the west. The procedings were conducted in a heated atmosphere of confrontation and recriminations. It was the third of the Ecumenical Councils, and was chiefly concerned with Nestorianism.
According to the Council, Nestorianism overemphasized the human nature of Christ at the expense of the divine. The Council denounced Patriarch Nestorius' teaching as erroneous. Nestorius taught that the Virgin Mary gave birth to a man, Jesus Christ, not God the Logos. The Logos only dwelt in Christ, as in a Temple (Christ, therefore, was only Theophoros: the "Bearer of God.") Consequently, the Virgin Mary should be called Christotokos ("Mother of Christ") and not Theotokos ("Mother of God").
The Council decreed that Christ was one person, not two separate "people": fully God and fully man, with a rational soul and body. The Virgin Mary is Theotokos because she gave birth not to a mere man but to God as a man. The union of the two natures of Christ took place in such a fashion that one did not disturb the other.