Mary was called the Mother of God from very early in the Church's history. One of her earliest titles in the liturgy is Theotokos which translates "God-Bearer". From this title she began to be called "Mother of God". Among those who objected to this title was Nestorius, whose false ideas on the nature of Christ is labeled Nestorianism. Nestorius claimed instead that Jesus was born of Mary as human and not as Divine, thereby doctrinally dividing the nature of Christ. St. Cyril defended the title by affirming with the Church fathers that Mary did indeed give birth to the Second Person of the Holy Trinity, who is called Jesus, who is simultaneously and completely both God and man from his birth. The Council of Ephesus (431 A.D.) was called to settle the dispute between Nestorius and St. Cyril. The council rightly affirmed St. Cyril and declared Nestorius a heretic. Thus the dogma of Mary as the Mother of God was established at the Council of Ephesus. From the decree: "If anyone does not confess that Emmanuel is God in truth, and therefore that the holy virgin is the mother of God (for she bore in a fleshly way the Word of God become flesh), let him be anathema." The decrees of the Council were then approved by Pope Sixtus III. - www.catholiccompany.com