Craig's version of the moral argument goes like this:
1. If God does not exist, then objective moral values and duties do not exist. (Premise)
2. Objective moral values and duties exists. (Premise)
3. Therefore, God exists. (From 1 and 2)
Say what you want about the argument's truth or falsity, but I'm much more interested in something else. You see, according to Craig, the Good, which is God himself, is concrete. With that I have no qualms whatsoever. Nevertheless, Craig denies that abstract objects exist. Instead, he adopts a form of nominalism. With that in mind, how can the commands of God (the Good) also be concrete? A command isn't something tangible, so I'm at a loss as to how I'm to understand what Craig is really saying.
If God's commands don't exist, by virtue of their being abstract, then they are nothing more than useful fictions. It's difficult for me to reconcile this with how Craig could possibly support premise (2) of his argument. If moral values and duties are abstract objects, and abstract objects are nothing more than useful fictions, then how can moral values and duties exist?
My own view on abstract objects is conceptualism, which I think has the potential to save the moral argument. It's entirely possible that I'm missing something. Craig is a much smarter philosopher than I am, and I'm willing to be corrected.