An excerpt from the work of St. Leo the Great, Pope and Doctor

The earth, Dearly Beloved, is indeed always, filled with the mercy of the Lord (Ps. xxxii. 5), and it is the teaching of nature itself to all who believe that God is to be adored, since heaven and earth, the sea, and all things that are in them, proclaim the goodness and might of their Creator. And the wondrous beauty of the stars, and of all creatures that serve Him, call for giving of thanks from that creature who is endowed with reason.  And now a more earnest purification of our hearts is asked of us, as once again we draw near to those special days in which we solemnly recall the mystery of man’s redemption, and which in ordered sequence precede the Paschal Festival. 

For though life of many among you is at all times without stain, and the regular practice of good works commends many of you to God, yet not for this reason are we so to trust to the healthiness of our conscience, that we may think that, though we dwell in the midst of temptations and occasions of sin, we cannot fall into anything which could injure it.  For the most worthy prophet says, Who can say, my heart is clean; I am free from sin? (Prov. xx. 9).  And again: Who can understand sins? From my secret ones cleanse me, O Lord:  and from those of others spare thy servant (Ps. xviii. 13). 

But if, as experience proves to us such is the state of the soul of those who resist evil desires, who struggle against the impulses of anger, and who restrain their most secret thoughts, so that never is anything found in their hearts that may be blamed, and yet are often unaware of their own secret sins, or troubled by those of others, let them then carefully consider during this time what are the blemishes, what the weaknesses, how grievous the effects of past sins, for which a stronger medicine may be needed: that they may not be found strangers to that mystery by which the works of the devil are destroyed (I Jn. iii. 8).