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

[MindForgedManacle]

Yes, those changes are called corrections based on things newly discovered in nature. Not because of changes in individual, cultural, or historical beliefs.

What? These things were the result of cultural and historical changes. And what logical system one should employ will be selected based on your own opinion about what you're applying it to. Further, moral realists could easily claim they're corrections too and not simply opinion, but I'll get to that below.

Moral rules are not based in nature but in human opinion.

So are the rules of logic and maths. What's your point?

You say that stealing is wrong, but what observation in nature leads you to assert this as objectively wrong?

I didn't say it was in wrong in all circumstances. Here, you're equivocating between "objective" and "absolute".
As a consequentialist, theft is wrong on my view if it decreases well-being, which it most often does.

Murder is always wrong because murder, by definition, is wrong. You probably mean to say "killing" instead of "murder".

No, murder is an unlawful killing, and I meant murder. Murder is not by definition wrong, just unlawful. But smoking weed is illegal too, doesn't mean it's wrong morally.

In that case, no, moral systems do not always accept killing to be wrong.

Many deontological moral frameworks do actually treat killing as immoral in all situations, just do a search for responses to Immanuel Kant's ethical framework.

You're assuming there is this "actual fact" that provides an objective basis for why slavery is wrong.

Well yes, as a consequentialist and a moral realist I DO have such an actual fact: Slavery causes a massive decrease in peoples' well-being. And given that goes against my moral axioms and is based on a demonstrable fact about the world (harm to the slaves), it is "objective".

Under moral subjectivism, the distinction between moral ontology and epistemology is a red herring.

Okay? It would be silly to say that is true for moral realism just because it is for a subjectivist.

Also, Occam's razor does not support extra unnecessary features/entities, so moral subjectivism would be more reasonable.

Er, no. First demonstrate that moral realism is an "extra, unnecessary feature". Just assuming that it is, is kind of silly.