those that do say so because they have this idea that what their god says is it, never changeable. So rape under circumstances is for them still fine. As would slavery. So clearly there is a problem with their world view.
First, if objective morality does exist (something they have yet to prove), that doesn't mean it derives from a god. There could be other reasons why it is objective. Tautologies are objectively true and don't require a god.
Secondly, the objective morality argument fails because if a god created morality, then it is arbitrary, not objective. If a god says something is good because it is an objective, then it is external to that god, not created by it.
Third, the god of the OT is most definitely immoral, and following the logic of the NT and many modern day Christians, the god of the NT is also evil. If atheists don't believe because Satan has blinded us, that means not only has this god allowed this, but to punish us for all eternity for something that Satan has allegedly done is also immoral. And eternal punishment for merely not believing, how on earth is that moral?
Science tells us we are born with an evolved, innate sense of morality that allows us to live in groups. It's called empathy. This is tempered by societal norms, such as your parents, and the groups you belong to. So morality is evidence for evolution, not for a god.