Revised: 31 May A.D. 2000 Mary, Queen of All Saints, Mediatrix
Theotokos - Mother of God
From the June 1998 Our Lady of the Rosary Parish Bulletin: Q. & A.
Question: How can Mary be the "Mother of God? Wouldn't that require her to exist before God, and to be His creator?
Answer: The Church traditional ascribes the title "Theotokos" (God bearer) to the Blessed Virgin. It freely acknowledges that Mary is a creature who came into being through the natural process of human generation (but who did not inherit original sin). Mary was conceived and born just a few years before Christ; long after God created everything from nothing. The title "Mother of God" implies nothing about the existence of God in eternity.
It is clear from the Sacred Scriptures and Tradition that our Lord Jesus Christ is the second Person of the Blessed Trinity; He is "born of the Father before all ages ... true God from true God; begotten, not made, of one substance with the Father...."1
It is also true that our Lord assumed human nature in such a way that we can say He is truly human, just as He is truly God. The Council of Ephesus (AD 431), called to refute the errors of Nestorius2 defines what we call the "hypostatic union":
By uniting to Himself in His own Person a body animated by a rational soul, the Word [second Person of the Trinity] has become man.... The natures that are brought together into true unity are different; still, from both there is one Christ and Son; not as though the difference between the natures were taken away by their union, but rather both divinity and humanity produce the perfection of our one Lord, Christ, and Son by their inexpressible and mysterious joining into unity. It was not that first an ordinary human being was born of the holy Virgin, and then the Word descended upon that man; but in virtue of the union He is said to have undergone birth of the flesh from His mother's womb, since He claims as His own birth, the generation of His own flesh. Thus [the holy Fathers of the Church] have not hesitated to call the holy Virgin Mother of God.3
By virtue of the hypostatic union, Jesus Christ is truly God and man. He was such from the moment of His conception in the womb of the Blessed Virgin Mary by the overshadowing of the Holy Ghost. Mary contributed human materiality that would form His body over the nine months of her pregnancy -- nine months during which God Himself dwelled within her. At the conclusion of that pregnancy, she gave birth to the God-man, bringing Him into the world for all to see. Mary is rightly called "Mother of God" because all that a mother does for her child, Mary did for the God-man.
There is no intention, in calling Mary "Mother of God," to imply that she came before God or that His divine existence depends somehow upon her.
(1) Niceno-Constantiopolitan Creed. Denzinger 86.
(2) Nestorius was the Patriarch of Constantinople (AD 428), who taught that the two natures of Christ were independent of each other to the degree that one could speak of Mary as "Mother of Christ, but not of God."
(3) July 22, 431. Denzinger 111a