Question: How can Mary be the Mother of God, if God is eternal? Wouldn’t that mean she existed before God?
It’s a great question, and one that was addressed definitively at the Council of Ephesus in 431. Before we go there, though, it’s good to clarify what Catholics (and in fact most Christians) believe about God and the Trinity.
Catholics believe in the Holy Trinity, meaning that we have one God in three persons: God the Father, God the Son, and God the Holy Spirit. “The divine persons do not share the one divinity among themselves but each of them is God whole and entire.” (CCC 253) God the Son, otherwise known as Jesus Christ, is the person of the Trinity who was born to Mary and became man.
Also central to answering your question is the fact that Catholics believe that Christ was both fully God and fully human. This belief was challenged by various heresies through the early centuries of the Church. The Nestorian heresy, for example, said that Christ’s human and divine natures were separate, not united in one person. Nestorianism therefore rejected the title “Mother of God,” arguing that Mary was only mother to the human Christ and not to the divine Christ.
Ultimately, at the Council of Ephesus, Nestorianism was denounced. The Council affirmed that Jesus Christ was both fully human and fully divine, two natures in one person. Therefore, it is correct to call Mary “The Mother of God” (the Greek word, Theotokos, literally means “God-bearer.”)
So did Mary exist before God? No. But she did give birth to God the Son, the second person of the Trinity who, being human as well as divine, entered into human history at a specific time, born to a specific woman. In a way, then, the title “Mother of God” is a succinct way of stating our belief that Jesus Christ was truly God. As the National Conference of Catholic Bishops say in their letter Behold Your Mother: Woman of Faith, “Mary can be rightly called ‘Mother of God,’ not indeed in the blasphemous sense of having existed before God, but as an affirmation of the truth of the Incarnation.”