This is the problem with humanism. It boils down to something like this:
"We should judge all humans by a moral code which we totally didn't steal from the Christians. Also, there is no God and your actions only affect the here and now, because there is no afterlife."
"If my actions don't have any bearing on what happens after I die, then why shouldn't I steal that car I've had my eye on?"
"Well... just don't, okay? Now if you'll excuse me I have to go and practice tipping my fedora. I'm going to perform at the atheist convention, you know."
Either humans, by and large, have a moral code written into their conscience (this is what we call the "natural law" in Christianity) and there's an author of that moral code (God), in which case it makes sense for us to follow that moral code in order to be rewarded by its Author. Or humans don't have that moral code, in which case I have to wonder why I'm sitting in a house in a city and not running around a field with my mates looking to find another tribe so that I can rape their women and burn their village down. You can't say that we wrote that code ourselves, because you can find it in every society there is, from the Yorubas to ancient Rome. Either we somehow came up with seven of the Ten Commandments when we were, so to speak, raping and burning each other, or they were given to us by our Creator. You can't have it both ways.
