Q. Why do Catholics call Mary The Mother of God?
A. For the simple reason that Mary was the mother of Jesus and since Jesus was God we also say she was the Mother of God. Mary was chosen by God to give her flesh, in the incarnation of the Second Person of the Holy Trinity. And since Jesus was born of woman as a baby who grew up just like all human babies He needed a Mother to care for Him. Mary was the Mother of Jesus, the God-Man. As she nursed and cared for her child she mothered Jesus in His totality. She did not mother only the flesh of Jesus, that would be absurd. Jesus was both fully Man and fully God and Mary was His mother–The Mother of God.
Q. But is this a wise title for Mary since it falsely gives the impression, perhaps, that Mary preexisted God?
A. Perhaps some will jump to the wrong conclusion at first. But, people of good will, will seek more understanding. The Catholic Church cannot stop teaching the truth just because some people might get the wrong impression.
Besides we know we are on safe ground because in Sacred Scripture Mary is called the mother of my Lord by Elizabeth in Luke 1:42-45. “Mother of my Lord” could just as easily be translated Mother of my God. But the Jews would not speak the name of God and substituted “Lord” for His name. But the meaning is exactly the same.
Also, in those same verses Elizabeth prophesies that all ages will call Mary blessed. But, most Protestant churches fail to fulfill this prophecy and do not refer to Mary as the Blessed Virgin Mary as we do in the Catholic Church. They do agree that Mary was blessed by God to bear the messiah in her womb, but they never call her Blessed.
The history of this title of our Blessed Virgin goes back to the first few centuries of the Church when a heresy arose that denied the humanity and the deity of Jesus. This title: Mary the Mother of God affirmed in a short and pithy way the doctrine of the Incarnation. The smallest child could teach and affirm this complex doctrine with those simple four words.