III. The Incarnation was necessary to the taking away of sin
In order therefore that we might be called to eternal bliss from our original bond and from earthly errors, He came down Himself to us to Whom we could not ascend, because, although there was in many the love of truth, yet the variety of our shifting opinions was deceived by the craft of misleading demons, and man's ignorance was dragged into diverse and conflicting notions by a falsely-called science. But to remove this mockery, whereby men's minds were taken captive to serve the arrogant devil, the teaching of the Law was not sufficient, nor could our nature be restored merely by the Prophet.' exhortations; but the reality of redemption had to be added to moral injunctions, and our fundamentally corrupt origin had to be re-born afresh.
A Victim had to be offered for our atonement Who should be both a partner of our race and free from our contamination, so that this design of God whereby it pleased Him to take away the sin of the world in the Nativity and Passion of Jesus Christ, might reach to all generations: and that we should not be disturbed but rather strengthened by these mysteries, which vary with the character of the times, since the Faith, whereby we live, has at no time suffered variation.
Source: Sermons (New Advent)