If we say that we have not sinned, we make him a liar, and his word is not in us. 1 John 1:10
"If you say there is no sin you are calling God a liar," which is outright denying God! Some people deny God for that reason, in an attempt to make their sins acceptable by denying them! The fact is, the denial of sin does not eliminate it. Only confession of sin can do that. Just like denial of God does not eliminate Him. In fact the denial of anything in itself does nothing by way of eliminating it.
Denial of sin, being a denial of the truth of God is simply a sin of the most basic kind: i.e. against the first commandment. (Why do you think it is the first commandment anyway? It was not invented yesterday!) "I am the Lord your God..." Whoever denies that denies God. Nietzsche himself (the apex of modern "enlightened" atheism) learned the tandem denial of Truth and of God from the avowed atheists of antiquity, against believers of every age!
Just a thought sparked by the Mother Angelica Classics show I saw last night on the EWTN television network. She herself was considering this fact: "If you say there is no sin you are calling God a liar." Better not to do that!