Denial of Sin is The Most Grievous Sin

The positive denial of sin is the whore daughter of her superlatively more profligate mother: atheism.

Every atheist is compelled to attempt to deny the reality of sin, the guilt of which no man can actually escape! for, every man is a sinner, yes, even the atheist who says that sin is not. That's his first sin, and the most serious of all. It implies that there is no truth, no justice and therefore, no real God! If you deny sin, you deny God.

Related to this, the atheist thus makes himself independent Lord of Truth! The atheist makes himself into God.

The very crime for which our Blessed Lord was accused, condemned and killed: crucified,...yesterday, Good Friday.
"We have a Law, and according to that Law he must die, because he made himself Son of God." John 19:7
However, Christ truly is God, the Way, the Truth and the Life, and there is no other!

Hear it, atheists! Christ died unjustly for the sin of which you are most guilty, your most grievous sin, your very denial thereof. Therefore, "Repent and believe in the gospel!" (Poenitemini, et credite Evangelio!) Mark 1:15

Confession is the cure! Confession is especially adapted as the cure for disbelief. The very act of confession (like any act of sincere worship) is an act of faith!

Confess your sins to a rightly ordained priest, as the only sure remedy for the most grievous sin of our age! the denial of sin. And hear the words of Christ, the greatest words men can utter, the words and the power of God Himself: "I absolve you from your sins in the name of the Father, and of the Son, + and of the Holy Spirit. Amen."

It's the forgiveness of God of which every man is in dire need! Every man needs to acknowledge and to confess his sins; and most of all, the resulting forgiveness of the priestly absolution and penitence!

N.B. The Parson's Tale (From Chaucer's Cantebury Tales)