Fr. Knox on why God became a man -- from The Belief of Catholics:
The hope of eternal life was not denied to fallen man, but it was offered, now, only as the prize of a severe probation. And he must struggle against an internal enemy he found too strong for him, with only such crumbs of uncovenanted assistance as God's mercy might afford. It was not intended, in God's Providence, that this pitiful condition of things should endure as long as the world lasted. Man's fault had been foreseen, and with the fault the Remedy. God became Man in order that, dying, he might atone for our sins, and win us the graces normally necessary to the attainment of salvation.
The coming of our Lord was thus not merely a Revelation to illuminate our minds; it was also designed to rescue man from his impoverishment and his spiritual dangers. It was to win for us, not only those "actual" graces by which, since then as before, God has turned our hearts to himself, but "habitual" grace, the state of "justification," in which we are assured of God's friendship, are enabled, during our lifetime, to perform actions pleasing to him, and at our death, if we have persevered, to attain the felicity of heaven. To achieve such blessings for us, it was needful to make amends for the affront offered by the sin of our first parents to the outraged Justice of Almighty God. Although he could have accepted some lesser sacrifice, he determined to make atonement for us himself, and to make it in full measure by the perfect offering of Death.
The Second Person, then, of the Blessed Trinity became Man for our sakes. Without losing or laying aside the Divine Nature which is his by right, he united to his own Divine Person a second, human Nature, in which he was born, lived on earth, and died. Once more the stubborn tradition of the Church could not rest content until it had fortified itself within these safeguards of definition. To think of Our Lord's Divine Nature as being annihilated, even temporarily, would be nonsense. A mere limitation of it, if that were thinkable, would not make it become truly human. To deny the reality of the human Nature would be false to all our evidence. Nothing less than a personal identity between the Eternal Word and Jesus of Nazareth would constitute a Divine Witness, or a Divine Victim. Every possible substitute for the received doctrine has been tried, and found wanting.
We believe that the circumstances of our Lord's coming into the world were marked by two miracles especially. In the first place, that she who was to be his Mother was endowed with that same gift of innocence which had been possessed and lost by our first parents; and that this freedom from the curse and the taint of "original sin" was bestowed upon her in the first instant of her Conception. And we also believe that both in and after the Birth of our Lord she remained a pure Virgin. From her, nevertheless, our Lord took a true human Body, which was the receptacle of a true human Soul. And in this human Nature he lived and died and rose again; and at last ascended into heaven, where it still persists.
Christmas, the Great Feast of the Son of God Who appeared in human flesh, the feast in which heaven stoops down to earth with ineffable grace and benevolence, is also the day on which Christianity and mankind, before the Crib, contemplating the "goodness and kindness of God our Saviour" become more deeply conscious of the intimate unity that God has established between them.
The Birth of the Saviour of the World, of the Restorer of human dignity in all its fullness, is the moment characterized by the alliance of all men of goodwill. There to the poor world, torn by discord, divided by selfishness, poisoned by hate, love will be restored, and it will be allowed to march forward in cordial harmony, towards the common goal, to find at last the cure for its wounds in the peace of Christ.
- From the 1944 Christmas Message of His Holiness, Venerable Pope Pius XII
Hodie Christus natus est!