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

[MindForgedManacle]

The point is that morality is based on standards that differ with individual, culture, and time.

The rules of mathematics and logic are thought up by rational beings like us, but they are at least based on nature and empirical observations. They do not differ with individual opinions, cultural beliefs, or zeitgeist of the time.

I wonder what systems of logic and inference rules you've been looking at, because those have most definitely changed over time. Logic isn't one thing, it's a multiplicity of axiomatic systems. There are commonalities amongst various logics but the views and applications of these have changed throughout the ages, and then one can say the same of morality. Logics tend to (but not always) accept the law of non-contradiction, and moral systems to accept that murder is wrong.

Also, when William Lane Craig argues about a distinction between moral ontology and moral epistemology, I find this to be a big red herring because the nature of morality is dependent on what you believe about it anyway. So there may be a distinction in meaning, but in the case of morality, is not a good counter argument to keep using whenever someone brings up moral issues that were once condoned in the past (such as slavery).

How is the nature of morality "dependent on hat you believe about it"? Under moral realism, that's false by definition. And yes, it is a good counter argument because you're confusing what people once thought about what they did with the actual fact of the matter about what they did.