The Council of Ephesus, which was held in 431, was crucial in affirming the truth of the title “Mother of God.” People had been calling Mary that for quite some time, but it was not dogmatically defined until the Council.
Here’s how it happened: At the Council, the bishops denounced the Nestorian heresy, a heresy that claimed that Christ’s human and divine natures were separate. Nestorians therefore believed that Mary was only the mother of the human Christ, not of the divine Christ.
In denouncing Nestorianism, the bishops affirmed the unity of the divine and human natures of Christ. Given that one cannot separate Christ’s divine nature from his human nature, the Council declared that it is correct to call Mary by the title of Theotokos, Greek for “God-bearer” – or, as we are more likely to say these days, “Mother of God.”