Atheeists: who told you sin is not sin?

I do not think it is false. However, it is connected to God and Christianity. God decides what is and is not a sin. Atheists do not believe in God. So we do not really recognize sin as something good or bad since we see no reason to believe the judgement of sin comes from the creator of the universe.

We understand sin is behavior that God condemns. We just do not believe in God so there is no sense in worrying about sin.