Sin is doing what is wrong before God. It is breaking His law (1 John 3:4). But, to say there is no such thing as sin is to say there is no God. But, if there is a God, then it makes sense to say that He is the Law giver, the one who reveals what is right and wrong like do not murder, do not steal, etc. Are you willing to bet your life that there is no God? Are you absolutely sure that how you have behaved in this world will mean nothing when you face God in the next?
Think about it. Saying there is no such thing as sin doesn't mean there isn't any. Besides, if you have you ever lied, or cheated, or stolen, then according to the Bible, you've sinned.
How do you "know" there is no such thing as sin? After all, you'd have to "know" there was no God, too. Are you sure you "know" there is no God . . . . or do you just "believe" there is no God? There is a big difference.