"The book of the genealogy of Jesus Christ"
It is of no avail to say that our Lord, the Son of the blessed Virgin Mary, was true and perfect man unless we believe that he is so in the way that the Gospel declares. For Matthew says: “The book of the generation of Jesus Christ, the son of David, the son of Abraham” and follows the order of his human origin so as to bring the lines of his ancestry down to Joseph, to whom the Lord's mother was espoused. Whereas Luke, going backwards step by step, traces his succession to the first of the human race to show that the first Adam and the last Adam were of the same nature (3,23f.).
No doubt the Almighty Son of God could have appeared for the purpose of teaching and justifying in exactly the same way as he appeared, in the semblance of flesh, to the patriarchs and prophets: for instance, when he wrestled with Jacob (Gn 32,25) or engaged in a conversation with Abraham, not refusing his hospitality and even partaking of the food set before him (Gn 18). But these appearances were indications of that man whose reality they manifested, assumed from the stock of those same ancestors.
But the fulfilment of the mystery of our redemption, ordained from all eternity, was not assisted by any images because the Holy Spirit had not yet come down on the Virgin and the power of the Most High had not overshadowed her (Lk 1,35). Wisdom had not yet built herself a house within her undefiled body so that the Word might there become flesh and, the form of God and the form of a slave coming together in one person, the Creator of time might be born in time and he himself, through whom all things were made, might be brought forth in the midst of all things. For if the New Man had not been made in the likeness of sinful flesh and taken our old nature on himself and, being consubstantial with the Father, had deigned to be consubstantial with his mother also – yet without sin – the whole human race would be held captive under the devil's yoke and we should not be able to make use of the Conqueror's victory because it would have been won outside our nature. But it was from Christ's marvellous sharing of our nature that the mystery of regeneration shone upon us.