Why Mary Is Truly the Mother of God Martin Luther. John Calvin. Zwingli. The very fathers of the Protestant Reformation insisted that Mary be called the Mother of God. Why? Because this title is not about Mary—it is about who Jesus is. When the early Christians called Mary the Mother of God, they were defending a core truth of the faith: Jesus is not partly God and partly man. He is fully God and fully man, one divine Person. That is why Elizabeth cried out, “How is it that the Mother of my Lord