I was reading Wes Morriston's 'God and the ontological foundation of morality', and I came across this brilliant objection to Craig's moral argument:
On divine command theory, our moral obligations turn out to be metaphysically identical to God's commands. In other words, facts about moral obligation are actually just facts about what commands God has issued to us, or what expectations he has of us. And so if God were to command some horror like rape, then it would become a moral obligation. But this is quite obviously not the case; even if God were to command us to rape, it would still be a morally reprehensible action—one that we are all obligated to avoid. This is among the most primal of our moral intuitions, and it's exactly these intuitions that Craig appeals to as support for the second premise of his moral argument. This is where Craig's account of moral value becomes crucial, as it gives him a way out of this problem. On Craig's view, God is essentially good and perfectly exemplifies every good thing. Craig notes that a perfectly loving, kind, just, ect., person simply would not command rape, or expect someone to rape, or to commit any similar atrocity. And so by grounding objective moral values in God's divine nature, Craig is able to say that there's no metaphysical possibility of God's issuing any such command.
And so "If God were to command rape, then it would be obligatory" is what is sometimes called a counterpossible, which is a counterfactual with an impossible antecedent. Craig then goes on to say that the popular view of counterpossibles is that, if they are true at all, they can only be vacuously true. The idea behind this is that, because the antecedent is impossible, it anything can follow from it. And so it would both be true that "were God to command rape, it would be obligatory" and "were God to command rape, it would not be obligatory". And so, as Craig argues, theists need not worry about counterpossibles as they have no informative power, and can't be used to show that divine command theory fails.
Craig's answer to this is to point out that, because the antecedent is necessarily false, the theist doesn't need to worry about it. Because impossible antecedents are vacuously true, in that the antecedents entail everything. It would be, according to Craig, both the case that "If God were to command rape, then rape would be good" *and* that "If God were to command rape, then rape would not be good".
But notice that "If God were to not exist, then objective moral values and duties would not exist" is *also* a counter-possible counterfactual! On theism the non-existence of God is a metaphysical impossibility, just as God's commanding a horror is a metaphysical impossibility. And so given this semantic theory of counter-possible counterfactuals that Dr. Craig has committed to, to avoid the defeat of divine command theory, the moral argument is completely undermined.
If counterfactuals with necessarily false antecedents are only vacuously true, then it's both the case that "If God did not exist, then objective moral values and duties would not exist" and that "If God did not exist, then objective moral values and duties would exist". And thus the atheist does not need to be concerned with first premise of Craig's moral argument, even if Craig was able to defend it (though Morriston goes on to show that all Craig's attempts are unsuccessful).