"What if someone says they’ve never lied, stolen, lusted, blasphemed—if they deny having any sin at all?"

Ask the person if he has kept the First of the Ten Commandments. Has he always loved God above all else—with all of his heart, soul, mind, and strength (Mark 12:30)? If he says that he has, gently say, "The Bible says that ‘there is none that seeks after God’ (Romans 3:11). Nobody (except Jesus Christ) has kept the First of the Ten Commandments. One of you is lying—either you or God—and the Bible says that it is impossible for God to lie" (Hebrews 6:18; Titus 1:2).