ExB: If you once believed in intelligent design as the origin of everything then go over to believe chance as the origin then who is to say what is right and wrong?
Morals are used to protect a group, be it a community or a nation. Thus, they are arrived at by the group, and as the community grows and becomes more organized, they are codified as rules or laws.
After all, it is not difficult to imagine what would happen to a community where murder and theft were not outlawed. The community would break down as people were robbed and killed, until few were left. Their lives would be worse off, and their prospects for surviving long enough to reproduce would dwindle. A community that worked to limit destructive behavior would not have this problem. Thus, human communities selected for traits and ideas that made for better and safer lives.
Even many animal species follow behavioral rules that protect the group, which makes them more likely to grow and evolve further. This kind of behavioral framework will work much better than those that do not control destructive behavior.
I don't need a god or book to help me to understand why certain behaviors are preferred and others are not. The simplest reason (but not the only one, by far) for why murder, theft, or rape are bad are that I know I would not want to experience them. A society where these behaviors are outlawed is going to be in better shape than one where they are not. This is easy to recognize without divine intervention.