Referring to Mary as “Mother of God,” however, does not imply that she existed from all eternity (like God) or that she is the source of Jesus’ divine nature. Mary was and is a human being. She is the Mother of God because she gave birth to the God-Man, Jesus, “the Word made flesh” (John 1).
The reality of Mary’s divine maternity was proclaimed a dogma of the faith by the Council of Ephesus in 431, and this teaching contains two important affirmations:
1) Mary is truly a mother. Since Jesus had no human father, Mary contributed all genetic material to the formation of His human nature. As Pope John Paul II states in his encyclical Redemptoris Mater, “[Jesus] is the flesh and blood of Mary!” (see Catechism 485)
2) Mary conceived and bore the Second Person of the Trinity. Echoing the Nestorian heresy (which denied the inseparable unity of two natures of Christ in one Person), some Protestant Christians hold that Mary was the mother of Jesus’ human nature only. But a mother does not give birth to a nature; she gives birth to a person. Since Jesus is a divine Person, it is logical that Mary be called the “Mother of God” (in Greek, Theotokos), even if this mystery has aspects that exceed our human understanding.
As the Catechism of the Catholic Church (1994) teaches
Called in the Gospels “the mother of Jesus,” Mary is acclaimed by Elizabeth, at the prompting of the Spirit and even before the birth of her son, as “the mother of my Lord.” In fact, the One whom she conceived as man by the Holy Spirit, who truly became her Son according to the flesh, was none other than the Father’s eternal Son, the second person of the Holy Trinity. Hence the Church confesses that Mary is truly “Mother of God” (Theotokos). [CCC 495]
The word Theotokos also helps us to understand this teaching a little better. The word literally means “God bearer,” not “God generator.” To “generate” God would imply that one is His origin, but this cannot be true because God exists from all eternity. To “bear” God means to hold him in one’s womb. Historic Christianity (i.e., the Catholic and Orthodox churches) believe that Mary actually bore God (in the person of Jesus Christ) in her womb. Jesus didn’t “become God” when He left her womb.
To deny Mary’s divine maternity is to cast doubt on the reality of Jesus’ divinity. Mary’s divine maternity is, then, essentially a “Christological” dogma in that it affirms the divine Personhood of Jesus. To emphasize the profound importance of this teaching, the Church has restored the ancient feast of Mary, Mother of God on January 1.
Since we have been reborn as children of God in baptism and now share in the divine life through grace, Mary has become our mother as well. By drawing near to her as our mother, we draw near to Jesus Himself, the source of our salvation. This is why devotion to Mary is so essential to the life of the Christian, and why the Church encourages us to foster a greater love for the Blessed Mother in our lives.
One final point. It is interesting to note that two of the early Protestant leaders, Martin Luther and John Calvin, taught Mary’s divine maternity and even condemned those who denied this essential truth.
Note: January 1st, the solemnity of Mary, Mother of God, is a holy day of obligation.