I'm sure most here who've questioned or left behind their belief in god have encountered this question - without god, what basis can there be for morality? My exwife, when I admitted that I was an atheist, once asked me "If you don't believe in god, what's to stop you from going out and raping and murdering?" Previously, whenever I got this objection, I would tend to turn it around on the other person with a response along the lines of "If fear of god is the only thing that stops you from murdering, that makes me rather nervous to be around you." With my exwife I tried additional lines of reasoning, but I think I went way too deep into moral philosophy for her shutdown cult mind to follow.
I recently encountered another response to this objection in a book I'm reading (Rationality: From AI to Zombies - I highly recommend it) that struck me as beautifully obvious in hindsight (as great ideas often do). Basically it asks what makes you nervous that god's rule against murder might go away? If god is the arbiter of morality, and suddenly decided that murder was not only acceptable but good, or if god disappeared altogether, why would you worry about that possibility? If that happened, then murder would be moral and there would be no reason to worry about people going off and killing indiscriminately because that is a perfectly moral thing to do. The point here is that whatever makes the person worry about the possibility of ending prohibition on murder - that is the true source of their ethics, not god. By the mere fact that they ask the question, they demonstrate that god is not important to morality.
I'm past the point where I find myself in discussion with theists anymore but I thought this an elegant argument that might be helpful for anyone here that finds themselves arguing the point (either with people in the cult or with themselves).