I am not saying that just because someone is an atheist, they will cheat, steal, kill, etc, but you cannot say that most people who break the law in those serious ways turn around and say "I am a Christian". Some of them might believe in a god, but they don't really have a definite religion.
And yes, Christianity does allow repentance for almost any sin, but to understand that you would have to go into the whole idea of what the God of the Bible is like, and I'm too tired to get into that now. Maybe some other time we can dive deeper into this, but for now, I'm hitting the sack.