(And Relativism (and Epistemology))
I've been half-heartedly intending to say something about this reader e-mail at Sullivan's digs, but another one of his readers got around to making the important point.
When discussing matters like the grounds of morality with either relativists or those who advocate theistic ethics, keep the following in mind: they tend to employ illegitimate negative formulations of their central claims that trick their opponents into taking up unnecessarily difficult dialectical positions.
E.g., they'll tend to assert things like:
Atheists can't explain the grounds of morality
No non-theistic moral theory can account for the grounds of morality
or, in the case of cultural moral relativists:
There is no culture-transcendent ground of morality.
These claims are likely to be false, but that's beside the current point. The small but very important dialectical point to keep in mind is that each of these claims suggests but does not entail that the theory being defended--theistic ethics or CMR--can give a successful account of the grounds of morality. (Similar points apply in logic and epistemology, in fact.)
Such folks say no theory of a type other than mine can solve the problem, and sucker you into trying to defend your theory instead of criticizing theirs. But no theory of a type other than mine can solve the problem does not entail my theory can solve the problem. Even if it turns out to be true that no non-theistic or non-relativistic theory can solve the grounding problem for morality (or epistemology), that doesn't mean that theism or relativism can help.
In fact, they never help. All they do is make the problem worse.
So don't fall for this trick.