Thomas Aquinas, Compendium of Theology

We indicated above that the reparation of human nature could not be effected either by Adam or by any other purely human being. For no individual man ever occupied a position of pre-eminence over the whole of nature; nor can any mere man be the cause of grace. The same reasoning shows that not even an angel could be the author of man's restoration. An angel cannot be the cause of grace, just as he cannot be man's recompense with regard to the ultimate perfection of beatitude, to which man was to be recalled. In this matter of beatitude angels and men are on a footing of equality. Nothing remains, therefore, but that such restoration could be effected by God alone.

But if God had decided to restore man solely by an act of His will and power, the order of divine justice would not have been observed. Justice demands satisfaction for sin. But God cannot render satisfaction, just as He cannot merit. Such a service pertains to one who is subject to another. Thus God was not in a position to satisfy for the sin of the whole of human nature; and a mere man was unable to do so, as we have just shown. Hence divine Wisdom judged it fitting that God should become man, so that thus one and the same person would be able both to restore man and to offer satisfaction. This is the reason for the divine Incarnation assigned by the Apostle in I Timothy 1:15: "Christ Jesus came into this world to save sinners."