6 For see what He says; “If we say that we have no sin, we deceive ourselves, and the truth is not in us.” Consequently, if you have confessed yourself a sinner, the truth is in you: for the Truth itself is light. Your life has not yet shone in perfect brightness, because there are sins in you; but yet you have already begun to be enlightened, because there is in you the confession of sins. For see what follows: “If we confess our sins, He is faithful and just to forgive us our sins, and to purge us from all iniquity.” Not only the past, but haply if we have contracted any from this life; because a man, so long as he bears the flesh, cannot but have some at any rate light sins.
But these which we call light, do not make light of. If you make light of them when you weigh them, be afraid when you count them. Many light make one huge sin: many drops fill the river; many grains make the lump. And what hope is there? Before all, confession: lest any think himself righteous, and, before the eyes of God who sees that which is, man, that was not and is, lift up the neck. Before all, then, confession; then, love: for of charity what is said? “Charity covers a multitude of sins.” Now let us see whether he commends charity in regard of the sins which subsequently overtake us: because charity alone extinguishes sins.
Pride extinguishes charity: therefore humility strengthens charity; charity extinguishes sins. Humility goes along with confession, the humility by which we confess ourselves sinners: this is humility, not to say it with the tongue, as if only to avoid arrogancy, lest we should displease men if we should say that we are righteous. This do the ungodly and insane: “I know indeed that I am righteous, but what shall I say before men? If I shall call myself righteous, who will bear it, who tolerate?
Let my righteousness be known unto God: I however will say that I am a sinner, but only that I may not be found odious for arrogancy.” Tell men what you are, tell God what you are. Because if you tell not God what you are, God condemns what He shall find in you. Would you not that He condemn you? Condemn you. Would you that He forgive? Do you acknowledge, that you may be able to say unto God, “Turn Your face from my sins.” Say also to Him those words in the same Psalm, “For I acknowledge mine iniquity.”
“If we confess our sins, He is faithful and just to forgive us our sins, and to purge us from all iniquity. If we say that we have not sinned, we make Him a liar, and His word is not in us.” If you shall say, I have not sinned, you make Him a liar, while you wish to make yourself true. How is it possible that God should be a liar, and man true, when the Scripture says the contrary, “Every man a liar, God alone true”? Consequently, God true through Himself, you true through God; because through yourself, a liar.
Source: Homilies on the First Epistle of John (New Advent)