Mother of WHAT?!
The purpose of this title is to express a critically important truth about Jesus, not simply to say nice things about his mother.
Some people used to say that Mary was only the mother of Christ’s human nature, making it seem almost as if there were two Christs: one human, one divine.
In fact, there is only one Lord Jesus Christ. Mary gave birth to Jesus: a single person fully human and fully divine. In that sense Mary can thus be called Mother of God (Mother in the sense of giving birth to One who is God, not in the sense that she was the cause of God).
The expression “Mother of God” is therefore primarily an affirmation of the unity of the person of our Lord Jesus Christ.
One of the most important churches in the city of Rome was dedicated to this truth. The dedication of that church, the Basilica of St. Mary Major, is celebrated throughout the world on this day.
(from earlier posts)