Definition of the Dogma: The Catechism of the Catholic Church (CCC) 495 states, “Called in the Gospels ‘the mother of Jesus’, Mary is acclaimed by Elizabeth, at the prompting of the Spirit and even before the birth of her son, as ‘the mother of my Lord ’. [Lk 1:43; Jn 2:1; 19:25; cf. Mt 13:55; et al.]
In fact, the One whom She [Mary] conceived as man and by the Holy Spirit, who truly became her Son according to the flesh, was none other than GOD the Father’s Eternal Son, the second person of the Holy Trinity. Hence the Church confesses that Mary is truly the ‘Mother of God’ thus (Theotokos).” [cf Council of Ephesus (431): DS 251.]
The Apostle’s Creed states, in part, “I believe in God, the Father Almighty, Creator of heaven and earth; and in Jesus Christ, His only Son, our Lord: Who was conceived by the Holy Spirit, born of the Virgin Mary.”
The prologue of John’s Gospel states, in part, “In the beginning was the Word, and the Word was with God, and the Word was God, and the Word become Flesh".
So the Council of Ephesus, in 431 A.D., condemned the teaching of Nestorius with these words: “If anyone does not confess that the Emmanuel in truth is God and on this account the Holy Virgin is the Mother of God – since according to the flesh she brought forth the Word of God made flesh – let him be anathema.”
BTW this is the 3rd ecumenical council, all Christians should believe this.
Mary was truly the Mother of Jesus.
Her Son Jesus was one person.
She bore the Logos, the Second Person of the Blessed Trinity, according to His humanity. Which did not exist beofre this Divine Intervention by the Holy Spirit through Gods Will.
The person she conceived and gave birth to was fully God and fully man. thus two nature’s fully Divine and fully Human combined in “one” individual called Jesus Christ, the Son of GOD became FLESH.
The New Testament acknowledged that Mary was the Mother of Jesus. They speak of His relation to Mary (Luke 2:4-7) and show Him to be a man (Luke 24:39). They declare Him to be God (John 1:1-4; Romans 9:1-5).
Early Church Father’s…
“For our God, Jesus Christ, was conceived by Mary in accord with God’s plan: of the seed of David, it is true, but also of the Holy Spirit” – [Saint Ignatius of Antioch; Bishop of Antioch; ca. A. D. 107; Letter to the Ephesians]
“The Word Himself, born of Mary who was still a Virgin, rightly received in birth the recapitulation of Adam, thereby recapitulating Adam Himself.” – [Saint Irenaeus; Bishop of Lyons; ca. A. D. 140 – ca. A. D. 202; Against Heresies]
“The Virgin Mary … being obedient to His Word, received from an angel the glad tidings that she would bear God.” – [Saint Irenaeus; Bishop of Lyons; ca. A. D. 140 – ca. A. D. 202; Against Heresies]
"If anyone does not agree that Holy Mary is the Mother of God, he is at odds with the Godhead. If anyone asserts that Christ passed through the Virgin as through a channel, and was not shaped in Her both divinely and humanly, divinely because without man and humanly because in accord with the law of gestation, he is likewise godless. If anyone assert that His manhood was formed, to be clothed over afterwards with divinity, he too is condemned; for this were not a generation of God, but a flight from generation . . . But He is not two Persons! Far be it! . . .” [Saint Gregory of Nazianz; 2nd of the Great Cappadocian Fathers; Bishop of Sasima; ca. A. D. 330 – ca. A. D. 389; Letter to Cledonius, Against Apollinaris, A. D. 382;
Jesus is true God…“GOD”, and true man. He is ONE, Single Divine Person, the Second Person of the Blessed Trinity in Whom, by His Incarnation, are two natures, one Divine, one Human, inseperable. All Christians today accept this truth formalized and proclaimed as de fide Dogmas of the Catholic Church. Mary, a true mother, was and is the Mother of that one, single Divine Person and is thus to be acknowledged as the Mother of God. This teaching was proclaimed to insure that the Dogmas concerning the identity and nature of Christ are clearly understood.
When friends, neighbors, family or acquaintances object that Catholics deify Mary by this teaching, you can now explain the reason and reasonableness of the title. No, Mary does not pre-date God, she is His creature. But she does pre-date God’s Incarnation in Jesus Christ. And since Jesus is not two people, but one Divine Person, she is rightly referred to as the mother of that person. The Mother of God.
SO THEN…Jesus, Who is God, is the only natural-born son who “chose His mother.”
He had a plan for Her life, and She accepted it with Her fiat, Her “yes” given to the Archangel Gabriel at the Annunciation, sent by GOD… who greets Her “HAIL MARY”. For that we are eternally grateful and indebted to Mary, who was given to us to be Our Mother, by Her Son Jesus from one of His last sentences on the Cross.
And if anyone ever suggests to you that you love Mary too much, answer, “Oh no, I could not possibly love Our Lady too much, because I could never love Her as much as She is loved by Her Son!” [most of this could be found on Catholic Life]
Peace