I wrote this to someone yesterday.
The periodic chart is a way of describing the way the world is. As we’ve come to understand the nature of reality better, the periodic chart has changed to incorporate that knowledge – but it’s all based on how the world really works.
Morals are the periodic chart of behavior. They are a system for determining the best behavior based on how the world is, and as our understanding about reality increases, our moral rules have changed.
There are, of course, several competing hypothesis about what these moral rules are. They vary from culture to culture, so don’t act like yours is the only one in town.
All the atheist must realize is that if a world with moral rules would be better, then that’s all the motivation we need to make them up. If we see, based only on how the world works, that telling the truth, not stealing, not killing each other, etc., makes people happier, then that’s all we need in order to suggest that “good” people ought to do those things. I think we can very easily defend that the world really works in these ways.
The theist, however, bases their morality on the commands of a god. That god is an assertion about reality. It is an assertion that hasn’t been adequately defended in the whole of human history. The theist has a much more difficult claim to defend than, “people who steal create a lousier world and are generally unhappy themselves.”