Fr. John Hardon once said that bad Mariology leads to bad Christology. That is definitely true among those who reject calling Mary Mother of God, which comes from the more ancient title Theotokos, which means “God-bearer.”
Consider these words in a sermon from the popular Protestant pastor John MacArthur:
Roman Catholics . . . say she gave birth to God and thus is to be elevated and adored. She gave birth to God. That is a terrible misconception. She gave birth to Jesus in his humanity. She did not give birth to God. God was never born.
I understand how a Protestant might think God can’t be “born” if he equates birth with coming into existence, since God is eternal. But in order to understand how Mary is the mother of God, we need to define our terms.
By “God,” I mean a divine person. When Thomas said to Jesus, “My Lord and my God,” he was not saying, “My Lord and my Trinity.” Thomas was saying Jesus is a divine person—the infinite God made man. Likewise, when we say “God was born” or “God died on a cross,” we are not saying the Trinity was born or that the Trinity died on the cross, we are saying that a divine person was born and a divine person died—namely, the second person of the Trinity, God the Son, who became the man Jesus of Nazareth.
By “Mother,” I mean a human who conceives, gestates, and gives birth to another person. Being the mother of God does not mean that Mary created Jesus’ divine nature, or even his human soul. The Catechism of the Catholic Church says, “Every spiritual soul is created immediately by God—it is not ‘produced’ by the parents” (366). Instead, Mary gave the Son his human nature and cared for this divine person in her womb, just as every other human mother gives a human nature to a person, her son or daughter, who resides in her womb.
Saying Mary is the mother of God simply means that Mary conceived a person within her womb, and that person was the divine Second Person of the Trinity, God the Son. Mary also gestated that divine person in her womb as he grew and developed, and then she gave birth to that divine person, who had assumed a fully human nature.
Saying Mary is the mother of God is another way of saying that Jesus was God throughout his entire human existence. And this shows why this dogma is so important, and why it was officially proclaimed in the early Church at the Council of Ephesus. Namely, it is heretical to deny that Mary is the mother of God because, in doing that, you deny the divinity of Christ.
In the early Church, adoptionist heretics claimed that Jesus Christ was a human person who became divine later in life—at his baptism, or even at his resurrection. But this theology denies the Incarnation, or that God became man, and replaces it with the heresy that a man became God. To safeguard against this heresy, we call the Blessed Virgin Mary Theotokos, the God-bearer, or mother of God.
In response, the Protestant apologist Eric Svendsen claims in his book Evangelical Answers that “the Person of Jesus isn’t merely God, any more than the person of Jesus is merely man. Put another way, Mary gave birth to a person who is both God and man. She did not give birth to the pre-incarnate form of the Logos. It is proper to call Mary ‘the mother of Jesus,’ but not ‘the mother of God’” (106).
Sure, Mary didn’t give birth to anything “preincarnate,” because Jesus’ birth occurred after his Incarnation, when he became a human embryo in Mary’s womb. But Mary did give birth to the Word, the Logos. John 1:14 says, “And the Word became flesh and dwelt among us.” To say Jesus is a human person and a divine person can be easily twisted to say the Second Person of the Trinity, a divine person, changed in his person, at the Incarnation, to be become a composite human-divine person. But God is infinite being itself. He cannot change.
That’s why St. Cyril of Alexandria told Nestorius, “We do not say that his flesh was turned into the nature of the godhead, nor that the unspeakable Word of God was changed into the nature of the flesh. For he, the Word, is unalterable and absolutely unchangeable and remains always the same as the Scriptures say” (Third Letter to Nestorius).
A critic who denies the dogma of Mary’s divine maternity is really in a pickle. If he says Mary was only the mother of Jesus’ body or his human nature, then I would ask, “Where was the Second Person of the Trinity, God the Son, when Mary was pregnant with fetal Jesus?” If he says God the Son was identical to the fetus in Mary’s womb, then he has to affirm that Mary is the mother of God. If that fetus is God, and being pregnant with a human fetus means you are a mother, then Mary is the mother of God. End of discussion.