The person easiest to fool is yourself. Many sincere people are honestly self-deceived.
Self deception is a self- inflicted blindness.
At least not until he is honest enough to acknowledge that he is engaged in self-deception. We need new life freed from bondage to sin. We are so depraved that, apart from the grace of God, we love lies more than truth. We have a natural aversion to truth but a natural attraction to lies. You must be born again.