Today I remembered Plantinga wrote a very interesting article on naturalism and obligation back in 2010 in the journal Faith and Philosophy. I just checked my archives and made an interesting discovery. First, that article reinforces my belief that Plantinga believes atheism (and, indeed, naturalism) and moral obligation are incompatible. Second, in that article, Plantinga explicitly says he is not going to argue directly for that incompatibility claim; rather, he intends to "display" the failure of the most natural way of arguing that naturalism can accommodate moral obligation. In his own words:
I propose to support the claim that naturalism cannot accommodate morality—not by showing directly that it can’t, but by displaying the failure of the most natural way of arguing that it can.
Imagine how Plantinga would have reacted if Mackie said, "I'm not going to directly argue for the claim that God and evil are incompatible. Rather, I'm going to indirectly argue for it by displaying the failure of the most natural way of arguing that God and evil are compatible." Surely Plantinga would have replied: "Even if Mackie succeeds at that task, that still falls short of what he needs to defend his logical argument from evil: a rigorous defense of the claim that God and evil are incompatible." And Plantinga would have been right.
By similar reasoning, then, it seems to me that the Autonomous Morality Defender ("Defender") would be just as correct to use a parallel reply to Plantinga. Even if Plantinga's article succeeds in showing that the "most natural way of arguing that" naturalism can "accommodate morality" fails, Plantinga would still fall short of what he needs to defend his 'logical' argument from morality for theism.
