I. We must always be seeking pardon, because we are always liable to sin
We proclaim the holy Fast of the Seventh Month, dearly-beloved, for the exercise of common devotions, confidently inciting you with fatherly exhortations to make Christian by your observance that which was formerly Jewish. For it is at all times suitable and in agreement with both the New and Old Testament, that the Divine Mercy should be sought with chastisement both of mind and body, because nothing is more effectual in prevailing with God than that a man should judge himself and never cease from asking pardon, knowing that he is never without fault.
For human nature has this flaw in itself, not planted there by the Creator but contracted by the transgressor, and transmitted to his posterity by the law of generation, so that from the corruptible body springs that which may corrupt the soul also. Hence although the inner man be now reborn in Christ and rescued from the bonds of captivity, it has unceasing conflicts with the flesh, and has to endure resistance in seeking to restrain vain desires. And in this strife such perfect victory is not easily obtained that even those habits which must be broken off do not still encumber us, and those vices which must be slain do not wound.
However wisely and prudently the mind presides as judge over the outer senses, yet even amid the pains it takes to rule and the limits it imposes on the appetites of the flesh, the temptation is always too close at hand. For who so abstracts himself from pleasure or pain of body that his mind is not affected by that which delights or racks it from without? Joy and sorrow are inseparable from a man: no part of him is free from the kindlings of wrath, the over-powerings of delight, the castings down of affliction. And what turning away from sin can there be, where ruler and ruled alike are liable to the same passions? Rightly does the Lord exclaim that “the spirit indeed is willing but the flesh is weak.”
Source: Sermons (New Advent)