Objection 1. It would seem that if man had not sinned, God would still have become incarnate. For the cause remaining, the effect also remains. But as Augustine says (De Trin. xiii, 17): "Many other things are to be considered in Incarnation of Christ besides absolution from sin"; and these were discussed above (Article 2). Therefore if man had not sinned, God would have become incarnate.
Objection 2. Further, it belongs to the omnipotence of the Divine power to perfect His works, and to manifest Himself by some infinite effect. But no mere creature can be called an infinite effect, since it is finite of its very essence. Now, seemingly, in the work of Incarnation alone is an infinite effect of the Divine power manifested in a special manner by which power things infinitely distant are united, inasmuch as it has been brought about that man is God. And in this work especially the universe would seem to be perfected, inasmuch as the last creature--viz. man--is united to the first principle--viz. God. Therefore, even if man had not sinned, God would have become incarnate.
Objection 3. Further, human nature has not been made more capable of grace by sin. But after sin it is capable of the grace of union, which is the greatest grace. Therefore, if man had not sinned, human nature would have been capable of this grace; nor would God have withheld from human nature any good it was capable of. Therefore, if man had not sinned, God would have become incarnate.
Objection 4. Further, God's predestination is eternal. But it is said of Christ (Romans 1:4): "Who was predestined the Son of God in power." Therefore, even before sin, it was necessary that the Son of God should become incarnate, in order to fulfil God's predestination.
Objection 5. Further, the mystery of Incarnation was revealed to the first man, as is plain from Genesis 2:23. "This now is bone of my bones," etc. which the Apostle says is "a great sacrament... in Christ and in the Church," as is plain from Ephesians 5:32. But man could not be fore-conscious of his fall, for the same reason that the angels could not, as Augustine proves (Gen. ad lit. xi, 18). Therefore, even if man had not sinned, God would have become incarnate.
On the contrary, Augustine says (De Verb. Apost. viii, 2), expounding what is set down in Luke 19:10, "For the Son of Man is come to seek and to save that which was lost"; "Therefore, if man had not sinned, the Son of Man would not have come." And on 1 Timothy 1:15, "Christ Jesus came into this world to save sinners," a 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 of medicine."
I answer that, There are different opinions about this question. For some say that even if man had not sinned, the Son of Man would have become incarnate. Others assert the contrary, and seemingly our assent ought rather to be given to this opinion.
For such things as spring from God's will, and beyond the creature's due, can be made known to us only through being revealed in the Sacred Scripture, in which the Divine Will is made known to us. Hence, since everywhere in the Sacred Scripture the sin of the first man is assigned as the reason of Incarnation, it is more in accordance with this to say that the work of Incarnation was ordained by God as a remedy for sin; so that, had sin not existed, Incarnation would not have been. And yet the power of God is not limited to this; even had sin not existed, God could have become incarnate.
Reply to Objection 1. All the other causes which are assigned in the preceding article 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 perfected by God with the righteousness of justice in order to know and carry out 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, on John 1:14, "And the Word was made flesh," St. Augustine says (Tract. ii): "Flesh had blinded thee, flesh heals thee; for Christ came and overthrew the vices of the flesh."
Reply to Objection 2. The infinity of Divine power is shown in the mode of production of things from nothing. Again, it suffices for the perfection of the universe that the creature be ordained in a natural manner to God as to an end. But that a creature should be united to God in person exceeds the limits of the perfection of nature.
Reply to Objection 3. A double capability may be remarked in human nature: one, in respect of the order of natural power, and this is always fulfilled by God, Who apportions to each according to its natural capability; the other in respect to the order of the Divine power, which all creatures implicitly obey; and the capability we speak of pertains to this. But God does not fulfil all such capabilities, otherwise God could do only what He has done in creatures, and this is false, as stated above (I, 105, 6). But there is no reason why human nature should not 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 (Romans 5:20): "Where sin abounded, grace did more abound." Hence, too, in the blessing of the Paschal candle, we say: "O happy fault, that merited such and so great a Redeemer!"
Reply to Objection 4. Predestination presupposes the foreknowledge of future things; and hence, as God predestines the salvation of anyone to be brought about by the prayers of others, so also He predestined the work of Incarnation to be the remedy of human sin.
Reply to Objection 5. Nothing prevents an effect from being revealed to one to whom the cause is not revealed. Hence, the mystery of Incarnation could be revealed to the first man without his being fore-conscious of his fall. For not everyone who knows the effect knows the cause.
Source: Summa Theologica (New Advent)