Where do morals, principles, ethics, whatever term one wishes to use, originate from? Theists of course always credit their respective deity with the origin and basis of ethics. [...] I would tentatively suggest that religiously justified morality with regard to slavery, becomes relative to whose interpretation you consider the most plausible. And, unsurprisingly, those Christians who supported slavery tended to side with Davis, those who did not sided with the abolitionists. The same is true in other issues outside of slavery. QED: Relativism is the default position in all cases in which God refuses to enlighten us - which happens to be all modern cases incidentally. And morals are acquired via cultural interaction and child development, which strengthens the provisional conclusion that morality is relative.
I think DS may be guilty of conflating the foundation of moral truths with that of our moral beliefs. How we learn about something is an entirely different matter from what makes it true. After all, our beliefs about (say) astronomy are likewise picked up in a cultural context. People used to think the sun went round the Earth; we now believe differently. That surely doesn't make astronomy 'relative'. Some beliefs are just plain false. Like, say, the belief that slavery is morally okay. People used to think that, and we might be able to explain this by appealing to their cultural context (or whatever), but this doesn't make the belief any less false.
DS argues that the existence of conflicting interpretations implies that religion cannot provide an objective basis for morality. But really this only shows that religion cannot provide an objective method for resolving moral disputes. In other words, it can't guarantee that we know the objective moral truths. But this isn't a particularly damaging result; I don't think many theists claim themselves to be morally omniscient, do they?
Practical problems aside, then, the thorny theoretical issue is not how we come to have moral beliefs, but rather, what makes them true (or false). It is here - as 'truth-maker', not 'belief-maker' - that many theists appeal to God. But this then raises the famous Euthyphro dilemma, the ultimate refutation of authority-based ethics:
Is X good because God commands it, or does God command it because it is good? If the latter, then moral standards exist independently of God. But the former is just a variant of subjectivism - moral truths are fixed by whatever God happens to prefer. If he liked slavery, then that fact alone would make slavery 'good'. Such arbitrariness makes for a pretty unsatisfactory moral foundation. Besides, it makes a mockery of calling God "good" - for that is just to say that God does what he likes, which doesn't seem a particularly praiseworthy characteristic. More sophisticated accounts argue that morality derives from God's character rather than his commands; but I don't see how that variation fares any better. To assess God in any meaningful way, we must appeal to a standard that is independent of him.
To wrap up: I agree with DS that our moral beliefs, like all our other beliefs, are culturally acquired. However, this implies nothing about what makes them true. In particular, it does not - in itself - support moral relativism. Even once we know the source of our beliefs, we are still left wondering about the source of moral truth (and normative 'force' in general). I think religious explanations fail here, but for different reasons than those proposed by DS. The question of relativism will receive further attention in my next posts...