2. Although in certain respects man is inferior to some other creatures, and in some respects is likened to the very lowest, yet in respect of the end for which he is created nothing is higher than man but God alone: for in God alone does the perfect happiness of man consist. This dignity of man, requiring to find happiness in the immediate vision of God, is most aptly shown by God's immediate assumption of human nature. The Incarnation has borne this fruit, visible to all eyes, that a considerable portion of mankind has abandoned the worship of creatures, trampled under foot the pleasures of the flesh, and devoted itself to the worship of God alone, in whom alone it expects the perfect making of its happiness, according to the admonition of the Apostle: Seek the things that are above (Col. iii, 1).
3. Since the perfect happiness of man lies in a knowledge of God beyond the natural capacity of any created intelligence (B. III, Chap. LII), there was wanted for man in this life a sort of foretaste of this knowledge to guide him to the fulness of it; and that foretaste is by faith (B. III, Chapp. XL, CLIII). But this knowledge of faith, whereby a man is guided to his last end, ought to be of the highest certitude: to which perfect certitude man needed to be instructed by God Himself made man. So it is said: No man hath seen God ever: the only begotten Son, who is in the bosom of the Father, he hath told us (John i, 18): For this I was born, and for this I came into the world to give testimony to the truth (John xviii, 37). Thus we see that since the Incarnation of Christ men have been instructed more evidently and surely in the knowledge of God, according to the text: The earth is filled with the knowledge of the Lord (Isai. xi, 9).
4. Since the perfect happiness of man consists in the enjoyment of God, it was requisite for man's heart to be disposed to desire this enjoyment. But the desire of enjoying anything springs from the love of it. Therefore it was requisite for man, making his way to perfect happiness, to be induced to love God. Now nothing induces us to love any one so much as the experience of his love for us. Nor could God's love for man have been more effectually demonstrated to man than by God's willing to be united with man in unity of person: for this is just the property of love, to unite the lover with the loved.
5. Friendship resting on a certain equality, persons very unequal cannot be conjoined in friendship. To promote familiar friendship then between man and God, it was expedient that God should become man, "that while we know God in visible form, we may thereby be borne on to the love of His invisible perfections " (Mass of Christmas Day).
6. For the strengthening of man in virtue it was requisite that he should receive doctrine and examples of virtue from God made man, since of mere men even the holiest are found at fault sometimes. I have given you an example, that as I have done so ye also do (John xiii, 15).
8. The tradition of the Church teaches us that the whole human race has been infected by sin. And it is part of the order of divine justice that sin should not be forgiven without satisfaction. But no mere man was able to satisfy for the sin of all mankind, since every mere man is something less than the whole multitude of mankind. For the deliverance then of mankind from their common sin, it was requisite for one to make satisfaction, who was at once man, so that satisfaction should be expected of him, and something above man, so that his merit should be sufficient to satisfy for the sin of the whole human race. Now in the order of happiness there is nothing greater than man but God alone: for though the angels are higher in condition of nature, they are not higher in respect to their final end, because they are made happy with the same happiness as man.* It was needful therefore for man's attainment of happiness that God should become man, to take away the sin of the world (John i, 29: Rom. iv, 25: v, 18: Heb. ix, 28).
4.46, 47 : That Christ was conceived by the Holy Ghost
4.55 : Points of Reply to Difficulties touching the Economy of the Incarnation