The Catechism of the Summa - Tertia Pars - XVI. OF CHRIST'S ENTRY INTO THE WORLD, VIZ., OF HIS BIRTH OF THE BLESSED MARY EVER A VIRGIN (B)

(B)
What is meant by saying that Jesus Christ was born of the Virgin Mary?

By this is meant that the Mother of Jesus, far from losing her virginity by becoming our Lord's Mother, rendered her virginity holy and doubly pleasing in the sight of God by becoming the Mother of our Lord; and this in such wise that, being a virgin before the conception of our Lord, she remained a virgin in the conception and during the birth, and for ever after the birth of our Lord (XXVIII. 1, 2, 3).

Were this conception and birth wholly miraculous?

Yes, it was wholly supernatural and miraculous that our Blessed Lady conceived and gave birth to our Lord in this manner; it must be remembered also that our Lady performed for her babe all such as any other mother performs with regard to her child born in the natural way (XXXI. 5; XXXII.).

Did God the Son unite the human nature to Himself in the womb of Mary together with all the gifts of grace, of which we have spoken, in an instant?

Yes, as soon as our Lady consented on the day of the Annunciation to become the Mother of God, immediately and in an instant by the action of the Holy Ghost she conceived in her virginal womb all the wonders that constitute the mystery of the Incarnation (XXXIII.-XXXIV.).

Must we say that from the first instant of conception our Lord had the use of His human free will, and that He began to merit already?
Yes, from the instant of His conception our Lord had in all their perfection both the beatific and infused knowledge of which we have already spoken; and He had the full use of His free will, and also began to merit by perfect merit such as is called de condigno (XXXI V. 1-3). Was the birth of our Lord a true birth in the sense that His Person was born, and how is this birth distinguished from that according to which we say He was born of the Father?

When it is said that God the Son was born of our Lady, by this is meant that the birth was a true birth affecting the Person of God the Son; but this birth is only spoken of in reference to Christ's human nature; whereas with regard to the Father He was born from all eternity, and this birth has reference to His divine nature (CXXV. 1, 2).

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