Objection 1. It would seem that it was not necessary for the reparation of the human race that the Word of God should become incarnate. For since the Word of God is perfect God, as has been said (I, 4, 1; I, 4, 2), no power was added to Him by the assumption of flesh. Therefore, if the incarnate Word of God restored human nature. He could also have restored it without assuming flesh.
Objection 2. Further, for the restoration of human nature, which had fallen through sin, nothing more is required than that man should satisfy for sin. Now man can satisfy, as it would seem, for sin; for God cannot require from man more than man can do, and since He is more inclined to be merciful than to punish, as He lays the act of sin to man's charge, so He ought to credit him with the contrary act. Therefore it was not necessary for the restoration of human nature that the Word of God should become incarnate.
Objection 3. Further, to revere God pertains especially to man's salvation; hence it is written (Malachi 1:6): "If, then, I be a father, where is my honor? and if I be a master, where is my fear?" But men revere God the more by considering Him as elevated above all, and far beyond man's senses, hence (Psalm 112:4) it is written: "The Lord is high above all nations, and His glory above the heavens"; and farther on: "Who is as the Lord our God?" which pertains to reverence. Therefore it would seem unfitting to man's salvation that God should be made like unto us by assuming flesh.
On the contrary, What frees the human race from perdition is necessary for the salvation of man. But the mystery of Incarnation is such; according to John 3:16: "God so loved the world as to give His only-begotten Son, that whosoever believeth in Him may not perish, but may have life everlasting." Therefore it was necessary for man's salvation that God should become incarnate.
I answer that, A thing is said to be necessary for a certain end in two ways. First, when the end cannot be without it; as food is necessary for the preservation of human life. Secondly, when the end is attained better and more conveniently, as a horse is necessary for a journey. In the first way it was not necessary that God should become incarnate for the restoration of human nature. For God with His omnipotent power could have restored human nature in many other ways. But in the second way it was necessary that God should become incarnate for the restoration of human nature. Hence Augustine says (De Trin. xii, 10): "We shall also show that other ways were not wanting to God, to Whose power all things are equally subject; but that there was not a more fitting way of healing our misery."
Now this may be viewed with respect to our "furtherance in good." First, with regard to faith, which is made more certain by believing God Himself Who speaks; hence Augustine says (De Civ. Dei xi, 2): "In order that man might journey more trustfully toward the truth, the Truth itself, the Son of God, having assumed human nature, established and founded faith." Secondly, with regard to hope, which is thereby greatly strengthened; hence Augustine says (De Trin. xiii): "Nothing was so necessary for raising our hope as to show us how deeply God loved us. And what could afford us a stronger proof of this than that the Son of God should become a partner with us of human nature?" Thirdly, with regard to charity, which is greatly enkindled by this; hence Augustine says (De Catech. Rudib. iv): "What greater cause is there of the Lord's coming than to show God's love for us?" And he afterwards adds: "If we have been slow to love, at least let us hasten to love in return." Fourthly, with regard to well-doing, in which He set us an example; hence Augustine says in a sermon (xxii de Temp.): "Man who might be seen was not to be followed; but God was to be followed, Who could not be seen. And therefore God was made man, that He Who might be seen by man, and Whom man might follow, might be shown to man." Fifthly, with regard to the full participation of the Divinity, which is the true bliss of man and end of human life; and this is bestowed upon us by Christ's humanity; for Augustine says in a sermon (xiii de Temp.): "God was made man, that man might be made God."
So also was this useful for our "withdrawal from evil." First, because man is taught by it not to prefer the devil to himself, nor to honor him who is the author of sin; hence Augustine says (De Trin. xiii, 17): "Since human nature is so united to God as to become one person, let not these proud spirits dare to prefer themselves to man, because they have no bodies." Secondly, because we are thereby taught how great is man's dignity, lest we should sully it with sin; hence Augustine says (De Vera Relig. xvi): "God has proved to us how high a place human nature holds amongst creatures, inasmuch as He appeared to men as a true man." And Pope Leo says in a sermon on the Nativity (xxi): "Learn, O Christian, thy worth; and being made a partner of the Divine nature, refuse to return by evil deeds to your former worthlessness." Thirdly, because, "in order to do away with man's presumption, the grace of God is commended in Jesus Christ, though no merits of ours went before," as Augustine says (De Trin. xiii, 17). Fourthly, because "man's pride, which is the greatest stumbling-block to our clinging to God, can be convinced and cured by humility so great," as Augustine says in the same place. Fifthly, in order to free man from the thraldom of sin, which, as Augustine says (De Trin. xiii, 13), "ought to be done in such a way that the devil should be overcome by the justice of the man Jesus Christ," and this was done by Christ satisfying for us. Now a mere man could not have satisfied for the whole human race, and God was not bound to satisfy; hence it behooved Jesus Christ to be both God and man. Hence Pope Leo says in the same sermon: "Weakness is assumed by strength, lowliness by majesty, mortality by eternity, in order that one and the same Mediator of God and men might die in one and rise in the other--for this was our fitting remedy. Unless He was God, He would not have brought a remedy; and unless He was man, He would not have set an example."
And there are very many other advantages which accrued, above man's apprehension.
Reply to Objection 1. This reason has to do with the first kind of necessity, without which we cannot attain to the end.
Reply to Objection 2. Satisfaction may be said to be sufficient in two ways--first, perfectly, inasmuch as it is condign, being adequate to make good the fault committed, and in this way the satisfaction of a mere man cannot be sufficient for sin, both because the whole of human nature has been corrupted by sin, whereas the goodness of any person or persons could not be made up adequately for the harm done to the whole of the nature; and also because a sin committed against God has a kind of infinity from the infinity of the Divine majesty, because the greater the person we offend, the more grievous the offense. Hence for condign satisfaction it was necessary that the act of the one satisfying should have an infinite efficiency, as being of God and man. Secondly, man's satisfaction may be termed sufficient, imperfectly--i.e. in the acceptation of him who is content with it, even though it is not condign, and in this way the satisfaction of a mere man is sufficient. And forasmuch as every imperfect presupposes some perfect thing, by which it is sustained, hence it is that satisfaction of every mere man has its efficiency from the satisfaction of Christ.
Reply to Objection 3. By taking flesh, God did not lessen His majesty; and in consequence did not lessen the reason for reverencing Him, which is increased by the increase of knowledge of Him. But, on the contrary, inasmuch as He wished to draw nigh to us by taking flesh, He greatly drew us to know Him.
Source: Summa Theologica (New Advent)