Nevertheless, let's just suppose that a rational person will either find something to ground morality in, or reject morality all together. This does not bode well for theism. Why? Because there are other things that the theist might want to believe in that have an air of queerness about them or intuitively call for grounding, like minds. And what is our currently best reductionist theory of mental states? It is that mental states reduce in some way to brain states. And so if the theist is going to charge the atheist with ignoring our currently best reduction of obligations to divine commands (supposing that is, in fact, the best current theory), then the atheist can just as easily retort that the theist is ignoring our currently best reduction of minds to brains. But if the theist were to even entertain this reductive theory of minds, he would have a very serious problem on his hands. God is, after all, an unembodied mind and thus on this theory would be impossible.
Now maybe there's some grounding principle the theist could come up with that is both plausible enough, and only calls for the grounding of morality and not for the grounding of minds, but I can't see what that could possibly be. Until the theist is able to flesh out such a principle, though, the question of "How do you ground objective morality?" will ultimately bite them back with even more force.