Bishop Gaume - Catechism of perseverance
Translated from: Gaume 1889, t. 3, pp. 282-284.
To conceive and to give birth constitute motherhood. Now, Mary conceived in her chaste womb Our Lord, the only-begotten Son of God, true God and true man, uniting in the person of the Word both divine and human natures.
Like all mothers, she formed His body from her own substance and virginal blood; she carried Him for nine months; and moreover, she gave birth to Him. Mary is therefore truly, and in the fullest sense of the term, the Mother of God. Mother of God, not that she gave birth to the divinity, which is too absurd an idea to have ever occurred to any Christian, but also not merely the mother of the flesh of the God-Man, any more than one says that other mothers are mothers only of the bodies of their children, although they do not generate their children’s souls; for they conceive and give birth to a human being composed of both body and soul.
In the same way, the Blessed Virgin is truly the Mother of God, because she conceived and gave birth to a God-Man, that is, a true man, formed from her substance and personally united to the divinity. Such is the explicit teaching of Scripture. You will conceive in your womb, said the Archangel to Mary, and you will bear a Son, and you will name Him Jesus. He will be great and will be called the Son of the Most High. Therefore, the one who will be born of you will be the Holy One par excellence, the Son of God 1.
Taught by the Holy Spirit about the mystery of the Incarnation, Saint Elizabeth exclaimed upon seeing Mary: And how is it granted to me that the mother of my Lord should come to me? 2. Summing up all these testimonies and others, Saint Paul says in his powerful language: The Son of God was made of the substance of a woman 3.
Such has always been, and still is, the faith of the Catholic world, solemnly defined at the Council of Ephesus held in 431, against the impious Nestorius.
“If anyone dares to deny that Our Lord is true God and true man, and consequently that the Holy Virgin is the Mother of God, let him be anathema!”
Never was an anathema more deserved. To deny the divine motherhood of Mary is to deny the unity of person in Our Lord Jesus Christ; it is to undermine Christianity at its foundation, for it is to destroy human Redemption.