Q. Why do we call Mary the “Mother of God?”
A. To understand, we have to look first at Jesus. From the very beginning, the Church has proclaimed that Jesus Christ is both God and Man. Jesus claimed for Himself the very name of God revealed to Moses, “I AM” (John 8:58), and He assumed divine prerogatives such as the forgiveness of sin (see Luke 5:18–26).
The Apostles testified to this reality. St. Thomas, for example, having known Jesus in His humanity, affirmed His divinity as well when he said to Him after His resurrection, “My Lord and my God!” (John 20:28).
St. John wrote in his Gospel that Jesus was “the Word” who “became flesh and dwelt among us,” and that this “Word was God” (John 1:1, 14). St. Paul taught that in Christ “dwells the whole fullness of the deity bodily” (Colossians 2:9).
When early Christians pondered these and other declarations of the apostolic witness, they wondered how exactly was Christ both human and divine? Was He, as some claimed, simply God and only appeared to be human? Was He, as others speculated, a human to whom God attached himself in a special way, dwelling inside Him? Or was He, as still others imagined, a kind of hybrid, partly human and partly divine?
Ultimately, in the light of Scripture and Tradition, and led by the Holy Spirit, the Church concluded that none of the above answers is correct. The Council of Ephesus, an ecumenical Church council held in the year 431, helped to resolve the issue. That council was provoked by a controversy over one particular question: Can we legitimately call Mary “the Mother of God”?
One prominent archbishop, named Nestorius, began to preach against the use of the Marian title Theotokos, which means literally “God-bearer,” or “the one who gives birth to God.” Christ was two persons, he claimed — one human, one divine — joined together in Christ. Though Mary was the bearer (or mother) of the human person in Christ, she was not the mother of the divine Person (God the Son). So she could not rightly be called the Mother of God.
After examining this teaching, the Church pronounced Nestorius mistaken. Christ was not a combination of two persons, one human and one divine. That would be close to saying that He was simply a man to whom God was joined in a uniquely intimate way — a man specially indwelled by God, like one of the Old Testament prophets.
Instead, the Church declared, Christ is only one divine Person — the Second Person of the Trinity. This single Person took our human nature and joined it to His own divine nature, so that He possesses two natures (Jn 1:1-3, 14).
But those natures don’t constitute two different persons. Christ is not a committee. The two natures belong to one and the same Person, the divine Son of God. And those two natures, though not to be confused, cannot be separated.
In this light, the Church concluded not only that it’s correct to call Mary the Mother of God, but that it’s important to do so. Mary conceived and bore in her womb the one Person, Jesus Christ, who is God in the flesh. If we deny that she is the Mother of God, then we are denying that her Son, Christ Himself, is God.