I think it's more likely that many theists truly believe that the only source of morality is from their notion of god and associated holy books.

The question I have for them is: "If you were to learn today for absolute certainty that there is no god, heaven, or hell, would that change your moral principles such that you could decide, for instance, that was okay to kill people or do whatever you want because there's no divine reward or punishment for your behavior?"

I suspect most religious people would reply, "No, of course not. Doing such things is just the wrong way to treat my fellow man." My morality comes from recognizing that there are lots of other people on the planet who want, and are entitled, to be treated as I want to be treated: the basic Golden Rule. It has nothing to do with god.