I. The Lenten fast an opportunity for restoring our purity.
In proposing to preach this most holy and important fast to you, dearly beloved, how shall I begin more fitly than by quoting the words of the Apostle, in whom Christ Himself was speaking, and by reminding you of what we have read: “behold, now is the acceptable time, behold now is the day of salvation.” For though there are no seasons which are not full of Divine blessings, and though access is ever open to us to God's mercy through His grace, yet now all men's minds should be moved with greater zeal to spiritual progress, and animated by larger confidence, when the return of the day, on which we were redeemed, invites us to all the duties of godliness: that we may keep the super-excellent mystery of the Lord's passion with bodies and hearts purified.
These great mysteries do indeed require from us such unflagging devotion and unwearied reverence that we should remain in God's sight always the same, as we ought to be found on the Easter feast itself. But because few have this constancy, and, because so long as the stricter observance is relaxed in consideration of the frailty of the flesh, and so long as one's interests extend over all the various actions of this life, even pious hearts must get some soils from the dust of the world, the Divine Providence has with great beneficence taken care that the discipline of the forty days should heal us and restore the purity of our minds, during which the faults of other times might be redeemed by pious acts and removed by chaste fasting.
Source: Sermons (New Advent)