Calling Mary the “Mother of God” is a simple syllogism: Mary is the Mother of Jesus. Jesus is God. Therefore, Mary is the mother of God. Logic!! Now, speaking of logic, this isn’t to say that Mary is somehow temporally before God, nor that she created God. Mary was not the mother of Jesus’ divine nature, but rather His human nature. God is outside of time and space (He created space-time!). Still, she wasn’t just the mother of Jesus’ human nature, she was the mother of Jesus Himself—the divine person! Jesus’ two natures were perfectly and uniquely united in His person. Hence, whatever we can truthfully say about Jesus, we can truthfully say about God.
So, for instance, we can rightly say that God suffered our death on the cross; God became incarnate; God wept at the death of his friend Lazarus; God Himself saved us; and so on.
This is important not only because of what it teaches us about God’s tenderness, humility, and closeness, but also because it preserves a correct understanding of the central figure of our faith: Jesus! Jesus was both fully God and fully man, and what we say about Mary helps to make this clear.
We thus have no problem focusing an entire solemnity on a specific title of Mary. Mary always helps us to understand her Son.
In case we are still unconvinced, though, just look at Elizabeth. Elizabeth, “filled with the Holy Spirit” exclaimed, “Why is this granted to me, that the mother of my Lord should come to me?” (Luke 1:41-42) Under the guidance of the Holy Spirit, Elizabeth didn’t exclaim, “That my Lord should come to me!” but, “That the mother of my Lord!” Clearly the Holy Spirit—aka God—has no qualms about guiding His people to get excited about Mary…and not only to get excited, but also to specifically call Mary the mother of my “Kyriou”, the mother of my divine Lord. It doesn’t get much clearer than that, so enjoy kicking off the new year just as the Holy Spirit would want you to, by celebrating Mary the Mother of our God!