The apologist William Lane Craig's moral argument goes something like this:
  1. If objective moral values and objective moral duties exist, then God exists.
  2. Objective moral values and objective moral duties exist.
  3. Therefore, God exists.

I've seen many people offer this argument, but I've never seen anyone offer an adequate defense of it, especially the first premise of the argument. A few comments on the "moral values" portion of premise 1 should help explain one reason why.

Premise 1 mentions "objective moral values". Moral values are whatever it is in virtue of which moral statements, moral beliefs, etc. [about what is morally good or morally bad] are true or false. I always found this easier to understand via an analogy to science. If someone says That plant is a mushroom, their statement is true (or false) in virtue of the features of the object their statement refers to. So that object's features/properties serve as the truth-makers for their statement. Parallel point for moral statements such as "that instance of rape was morally bad": moral values are whatever server as the truth-makers for those moral statements.
[This usage of "moral values" differs from another common usage of "moral values", where "moral values" means something like "a particular subset of stuff one cares about". For example, just as saying "I value my wife" means something like "I care about my wife" on this usage of "value", saying "human welfare is one of my moral values" means something like "human welfare is one of things I care about." This sense of "moral values" differs from the sense of "moral values" Craig is (and should) be using in his moral argument, as I discussed above. Confusing these two sense of "moral value" would be akin to confusing what "theory" means in science vs. what it means everyday life, or confusing what "root" means in plant biology vs. what it means in biology. In short: it would be an equivocation.]

So the first "moral values" portion of Craig's first premise commits him to saying something like God exists if the objective truth-makers of moral statements, moral beliefs, etc. [about what is morally good or morally bad] exist. And that doesn't make a lick of sense. After all, there are plenty on plausible of accounts of the objective truth-makers of moral statements, where those accounts are compatible with God's nonexistence. These include varieties of virtue ethics, deontology, and utilitarianism. For example, a welfare utilitarianist could note that the moral statement "that instance of rape is morally bad" is true or false in virtue of the rape's negative effects on the victim's welfare, where there were other viable actions that did not harm the victim's welfare in this way. Or a virtue ethicist can point out the callousness that lies behind the act.

Craig has to reject all such plausible options for objective moral values, since welfare, character traits, etc. can exist even if God does not, and Craig is committed to moral values implying God's existence. And in doing this, Craig commits himself to a wildly implausible subjectivist position: divine command theory.

Furthermore, Craig opts for a strange, implausible position where moral values must be grounded in God's nature. So, for instance, "that instance of rape is morally bad" is true or false not because of the harm by the rape nor the callousness of the act nor any such plausible answer, but instead because God has such-and-such a nature. I'm sorry, but that's laughable. That answer has nothing to with why the rape is morally bad. It would be like responding to In virtue of what is "that plant is a mushroom" true, with well, biological properties/features are grounded in God's nature. It's a nonsense answer. Biological properties/features are grounded in the particulars things (such as organisms) that have those properties, not God. So the feature referred to by "is a mushroom", is grounded in the particular referred to by "that plant". No God required. Parallel point for moral values as moral properties: they are grounded in the particular actions, persons, etc. that have those properties. So in the rape example, the moral values are grounded in features of the rape itself, not God. It's rather strange that Craig's argument commits him to denying something this apparent.