A number of false teachers in the days of the apostle John were deceiving themselves by denying the doctrine of sin. But the sin of man is made very clear by the witness of history, the Law of God and by our own consciences. If we deny this evidence, we are accusing God Himself of sin.
1John.1.8-10If we say that we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness. If we say that we have not sinned, we make him a liar, and his word is not in us.