Today is also the Solemnity of the Blessed Virgin Mary, Mother of God!
Mary is the Theotokos, which means "Mother of God" or "God-bearer."
This Dogma was defined at the one-day Ecumenical Council of Ephesus in 431 AD.
It was called to combat claims made my Nestorius, the founder of Nestorianism and also the Patriarch of Constantinople. He preached a duel personhood of Jesus Christ and said that Christ was two separate persons: one divine and one human. According to Nestorius, the Son of God was distinct from the Son of David.
No one would have known about Nestorius’ heretical views, except that he was unwilling to call Mary the Theotokos. He called her rather the Christotokos, meaning "human bearer of Christ" and not bearer of God.
St. Cyril of Alexandria, who presided at Ephesus, defended orthodox Christology and orthodox Mariology.
Nestorius came to the Council first with an army of men (for protection). He tried to ex-communicate St. Cyril in his absence. However, when St. Cyril arrived (with an army of naval men) he pronounced an ex-communication on Nestorius which actually had authority behind it.
St. Cyril was the champion, not only of Mary’s proper place in salvation history, but also of defending the Hypostatic Union of Christ which proclaims that Christ is one divine person with a fully human nature and a fully divine nature.
Jesus Christ is God, Mary is Mother of Jesus Christ, thus Mary is the Mother of God!