How did we come to call Mary the 'Mother of God'?

(OSV News) -- Jan. 1 is the solemnity of Mary, Mother of God, the final day of the Christmas octave.

In the fifth century, a heresy led by Nestorius, patriarch of Constantinople (r. 428-431) claimed that Jesus was actually two persons: one human and one divine -- that his divinity was instilled on him after he was born. Thus, they reasoned incorrectly that Mary was the mother of Jesus but not the mother of God. Their rationale contradicted ancient Christian beliefs as well as proclamations and canons issued at earlier Church councils.

At the Ecumenical Council of Nicea in 325, the Church Fathers had clearly determined that Jesus was consubstantial with the Father and, therefore, Mary was the Mother of God.

In response to the heretical message of Nestorius, another ecumenical council was held in 431 at Ephesus, Turkey. Led by St. Cyril (r. 412-444), bishop of Alexandria, Egypt, Mary was defended as Theotokos, God-bearer, and that Jesus was one person with a divine and human nature; Mary was the Mother of God. Nestorianism was condemned by the council and Nestorians excommunicated.

The people of Ephesus, joyful over the council decision, went through the streets chanting, "Mary, Mother of God," which would become words prayed during the rosary devotion.

Some 1,500 years after the council, Pope Pius XI (r. 1922-1939) would claim: "If the Son of the Blessed Virgin Mary is God, assuredly she who bore him is rightly and deservedly to be called the Mother of God."