In response to some new replies on the Ethics Without Metaphysics post. The link tracks back to the original post.
The argument you've given is very close Mackie's argument from queerness, posited in "Ethics: Inventing Right and Wrong." For Mackie, an error theorist, the argument purports to be a problem for all moral realists, whether they happen to be theists or atheists. Why is it a problem specifically for atheists?
Gordon Knight said:
The argument from queerness is a bad argument. By the same lights, mathematics and logical truth are "queer."
RD and Gordon: Yes, to my mind, a "matter-first, mind-later" ontological hierarchy is going to have trouble with the mathematics and logic, that's what is known as the argument from reason!
However, naturalism puts a restriction on what can be fundamental properties of objects in a naturalistic universe. Moral properties are not permitted. Mental properties are also not permitted. They have to be "system properties" that arise at a higher level of organization, when brains show up. However, while there is something incoherent about the idea of a piece of matter being intrinsically morally good, there is nothing about God being intrinsically morally good that is incoherent. So I think this is an asymmetrical problem that afflicts the naturalist but not the theist.