Feast of the Blessed Virgin Mary, Mother of God
The Dogma: Mary is truly the Mother of God (De fide).
The Church professes Her belief that Jesus Christ , as the Son of God, is also true God: the Jews sought all the more to kill Him, because He… also said God was His Father, making Himself equal to God (Jn. 5.18); We stone Thee… for blasphemy… because that Thou, being a man, makest Thyself God (Jn. 10.33); And the high priest said to Him: I adjure Thee by the living God, that Thou tell us if Thou be… the Son of God. Jesus saith to him: thou hast said it. Nevertheless I say to you, hereafter you shall see the Son of man sitting on the right hand of the power of God, and coming in the clouds of heaven. Then the high priest rent his garments, saying: He hath blasphemed…. He is guilty of death (Mt. 26.63-64,65,66).
As the Mother of the Son of God, therefore, Mary is the “Mother of God”. The Council of Ephesus with St. Cyril of Alexandria declared against Nestorius in his heresy [that “Christ was the Son of God not only by the grace of adoption but by the divine nature in which (the Person of the Son of God, united to the man-Christ by mere habitation in him – taking partly the error of Photinus) existed coeternal with the Father” – in St. Thomas Aquinas, Compendium of Theology]: “If any one does not confess that the Emmanuel in truth is God and that on this account the Holy Virgin is the Mother of God (“Theotokos”) – since according to the flesh She brought forth the Word of God made flesh – let him be anathema. The subsequent General Councils repeated and confirmed this doctrine.
The dogma of Mary’s Motherhood of God contains two truths –
1) Mary is truly a mother, that is, She contributed everything to the formation of the human nature of Christ, that every other mother contributes to the formation of the fruit of her body;
2) Mary is truly the Mother of god, that is, She conceived and bore the Second Person of the Divinity, not indeed according to the Divine Nature, but according to the assumed human nature.