Way back in 431, there was a bitter controversy among theologians over the role of Mary in the Catholic Church. They debated the question: Who is Mary in God’s plan?
In the end, the bishops declared that Mary is really the mother of Jesus and Jesus is really God. So it must be said that Mary is the Mother of God. The oldest Christian greeting of Mary was proclaimed when Mary’s relative Elizabeth called her “Mother of my Lord.” When Elizabeth welcomed Mary, she recognized both the great privilege God had given Mary and Mary’s great faith in accepting it.
For centuries, Mary has been praised because she believed. She is Mother of God because of her faith in God. The Solemnity of Mary, Mother of God, celebrates Mary’s faith and trust in God alone. The Church wants us to imitate her faith.
from Saints and Feast Days, by Sisters of Notre Dame of Chardon, Ohio