Protestants reject the fact that Mary is the Mother of God. Catholics know God's divinity didn't originate in Mary's womb. The Second Person of the Blessed Trinity did, however, unite Himself to a real human soul in Mary's womb and there clothed Himself with a real human body.
Before the third century, the Fathers of the Church had already given Mary the Greek title Theotokos, meaning God-Bearer. Because this term was disputed by heretics like Nestorius, the Church in 431 at the Council of Ephesus solemnly defined that it was theologically proper to call Mary the Mother of God.
Nestorius falsely claimed that Mary gave birth only to the human nature of Christ. At Ephesus, the Church asserted that Mary gave birth to the Person of Christ and not merely Christ's body. The divine person, Son of God the Father from all eternity, was the same divine person born of Mary. Although Mary is the daughter of God the Father and spouse of God the Holy Spirit, She's still the Mother of God the Son, which makes Her Mother of God.
The faithful have been invoking Mary's aid under the title Mother of God from time immemorial, as shown by the Angelic Salutation. The Hail Mary, recited by the faithful in the Rosary, contains the phrase: "Holy Mary, Mother of God, pray for us sinners." The traditional feast exalting Mary's maternity, however, was extended to the universal liturgical calendar in 1931 by Pope Pius XI, who set aside October 11 for its observance.
As God clothes a person's soul with human flesh in the womb of his mother, so too did Christ clothe Himself with flesh in the womb of His mother Mary. On the Cross, Our Lord extended Mary's maternity to all the human race when in John 19:26 He said to Her, "Woman, behold thy Son" and to St. John, "Behold thy mother."
Learn how Mary leads us to Christ in Church Militant's Premium show Basic Training—The Virgin Mary.