With or without religion, good people can behave well and bad people can do evil; but for good people to do evil—that takes religion. – Steven Weinberg
One of the biggest things you can say against cultural relativism is that, when you break it down far enough, pretty much every ethical system that human beings have ever created has been based on the same underlying principle. Every society has had as its moral goal the largest amount of happiness for all persons – we’ve just disagreed vastly on who counts as a person, and how to best achieve that happiness. This is why Utilitarianism was such an influential school of thought – it’s not so much that it was revolutionary or innovative; it’s that it finally codified and rationalised what we’ve been wordlessly striving for all along.