what are we supposed to say again when christians ask us where we get our morality?

[Esquilax]

It's simple: if god's moral commandments are actually based on reality and the welfare of the people in it, then those benefits can be discerned without needing a god to tell you them. If god's moral commandments aren't based on reality then they're just the opinions of god and, whether you worship him or not, there's simply no reason for them not to be another way; under that scheme, murder is qualitatively just as moral as anything else.