The first of January is not just New Years Day for Catholics. It is also the Solemnity of Mary, the Mother of God. It happens to fall on New Years not because the Catholic Church wanted to ring in the new year with Marian devotion, but because January 1 is the eighth day after Christmas. The Octave of Christmas, which began with the celebration of Jesus’ birth, ends with a celebration devoted to the woman who birthed Him. It seems fitting.

But if one scratches the surface of this feast, one discovers that — like all Marian devotions — it all has to do with Jesus. Consider this antiphon from today’s Morning Prayer in the Liturgy of the Hours:

Marvelous is the mystery proclaimed today: man’s nature is made new as God becomes man; he remains what he was and becomes what he was not. Yet each nature stays distinct and for ever undivided.

There is not a single word about Mary in this prayer. It’s all about the Incarnation of God. So what’s it doing in the Liturgy for the Solemnity of Mary, Mother of God? And what is all this talk about nature?

There is an old adage in the Church: lex orandi, lex credendi. Roughly translated, it means “the law of prayer is the law of belief,” or “as we pray, so we believe.” It is said that if you want to really know what the Church believes, all you have to do is look at the prayers of her liturgy. This helps to explain why this prayer, which is all about the divine and human natures of Christ, is included in the liturgy of Mary, Mother of God.

It all goes back to a fifth century heresy. Christians have always loved Jesus’ mother and honored her with special titles, one of the oldest of which is Theotokos, Greek for “God bearer.” It is usually translated into English as “Mother of God.” But in the early fifth century some began to suggest it was wrong to call Mary the Mother of God. They claimed it was permissible to call her the Mother of Jesus, or the Mother of the Christ, but not the Mother of God, because this could be taken to mean Mary, a human woman, was somehow the origin of the divine and eternal God, which cannot be true. Many agreed with this line of thinking.

But many more objected. Theotokos was one of the oldest and most beloved titles for Mary. On the level of popular piety, people were unwilling to give up their devotion to Mary as Mother of God. And on a theological level, many Church Fathers also objected to this criticism of the title.

To say that Mary is the Mother of Jesus but not the Mother of God would be to introduce a division into Person of Jesus Christ. It would mean saying that Mary is the mother of Jesus’ human nature only, and not also the mother of His divine nature. This could easily lead people to think there is a separation in Christ, that His divine nature was something added on to His humanity, or that existed alongside, but not in complete union, with it.

The Church Fathers could not tolerate this. This is the era when the Church began to clearly articulate the teaching of the hypostatic union. This doctrine asserts that the two natures of Christ, human and divine, are full and complete natures existing in the one Person of Jesus. This unity of Jesus’ Personhood ultimately is what made the difference in the debate over Mary’s title of Mother of God. Mothers don’t conceive and give birth to natures, after all. They conceive and give birth to persons. And the person Mary conceived and gave birth to is a Divine Person.

By calling Mary the Theotokos, we uphold a very important teaching about Jesus Christ. Mary is the mother of Jesus. Jesus is God. Therefore Mary is the Mother of God. And so the Council of Ephesus dogmatically declared in favor of this Marian title in 431 A.D.

Behind the Marian title is a Christological truth. When we celebrate this great Solemnity on the Octave of Christmas, and any time we give honor to Mary as Mother of God, we allow Mary to act in our lives in the way she always acts — to point the way to her Son. Her final recorded words in scripture, after all, are, “Do whatever He tells you” (Jn 2:5).