Augustine explaining Luke 19, says, “’The Son of Man is come to seek and to save which was lost’: — Therefore, if man had not sinned, the Son of Man would not have come. And in 1 Timothy 1:15: Christ Jesus came into this world to save sinners,’ the gloss says: There was no cause of Christ’s coming into the world, except to save sinners. Take away diseases, take away wounds, and there is no need for medicine.”

1. There are different opinions about this question. Some claim that even if man had not sinned the Son of God would have become incarnate. Others declare the contrary, and our assent ought rather to be given to this opinion.

For such things as come from God’s will, and beyond the creature’s due, can only be made known to us by being revealed in the Sacred Scripture, in which the Divine Will is make known to us. Hence, since everywhere in Sacred Scripture the sin of the first man is assigned as the reason of the Incarnation, it is more in accordance with this to say that the work of the Incarnation was ordained by God as a remedy for sin; so that, had sin not existed, the Incarnation would not have been. Although the power of God is not limited to this; even had sin not been, God would have been incarnate.

2. All other things which are assigned to the Incarnation of Christ, namely, man’s promoting in faith, hope, and charity, have to do with a remedy for sin. For if man had not sinned, he would have been endowed with the light of Divine Wisdom and would have been protected by God with the righteousness of justice in order to know and execute everything needful. But because man on deserting God had stooped to corporeal things, it was necessary that God should take flesh, and by corporeal things should afford him the remedy of salvation. Hence in John 1:14, “And the Word was made flesh,” Augustine says: “Flesh had blinded thee, flesh heals thee; for Christ came and overthrew the vices of the flesh.
3. On the other hand, it is reasonable and fitting that human nature should have been raised to something greater after sin. For God allows evils to happen in order to bring a greater good therefrom; hence it is written (Rom. 5:20): but where sin increased, grace abounded all the more.” Hence also, in the blessing of the Paschal candle, we say: O happy fault, that merited such and so great a Redeemer!” (Summa 3a, q. 1, a. 3.)