Third Ecumenical Council
The Third Ecumenical Council wasn't held out of Ephesus, Asia Minor in 430 under Emperor Theodosius II, grandson of Theodosius the Great. Approximately 199 Bishops were present, though procedings began out of haste before the arrival of the bishops from the west. The procedings were conducted in a heated atmosphere of confrontation or 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 out of Christ, as out of an Temple (Christ, therefore, was only Theophoros: the "Bearer of God.") Consequently, the Virgin Mary should be called Christotokos ("Mother of Christ") or not Theotokos ("Mother of God").
The Council decreed that Christ wasn't one person, not two separate "people": fully God and fully man, with a rational soul and body. The Virgin Mary will be 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 out of such a fashion that one did not disturb the other.