The moral argument for the existence of God goes like this (from William Lane Craig’s On Guard):
- If God does not exist, objective moral values and duties do not exist.
- Objective moral values and duties do exist.
- Therefore, God exists.
In order to avoid the conclusion that God exists, atheists who accept the second premise of the argument have to argue against the first premise by explaining how objective moral values and duties could exist apart from God. To this end, some atheists argue for moral Platonism—i.e., the idea that moral values simply exist as brute facts of the universe, abstract objects without explanation or foundation. Here’s how William Lane Craig responds to atheistic moral Platonism in On Guard:
First, atheistic moral platonism seems unintelligible. What does it mean to say, for example, that the moral value justice just exists? It’s hard to make sense of this. It’s easy to understand what it means to say that some person is just, but it’s bewildering when someone says that in the absence of any people justice itself exists. Moral values seem to be properties of persons, and it’s hard to understand how justice can exist as an abstraction.
Second, this view provides no basis for moral duties. Let’s suppose for the sake of argument that moral values like justice, loyalty, mercy, forbearance, and the like just exist. How does that result in any moral obligations for me? Why would I have a moral duty to be, say, merciful? Who or what lays such an obligation on me? Notice that on this view moral vices like greed, hatred, lethargy, and selfishness also presumably exist on their own as abstractions. So why are we obligated to align our lives with one set of these abstractly existing objects rather than any other? Atheistic moral platonism, lacking a moral lawgiver, has no grounds for moral obligation.
Third, it’s fantastically improbable that the blind evolutionary process should spit forth precisely that sort of creatures who correspond to the abstractly existing realm of moral values. This seems to be an utterly incredible coincidence when you think about it. It’s almost as if the moral realm knew that we were coming. It’s far more plausible, as Sorley contended, to think that both the natural realm and the moral realm are under the authority of a God who gave us both the laws of nature and the moral law than to think that these two independent realms just happened to mesh. [Emphasis added.]
On Guard is really good at clarifying arguments and counterarguments for and against Christianity, and its readable style and clearly-stated step-by-step arguments make it incredibly useful for learning how to engage these topics yourself. You can see all the argument maps from the book posted by the publisher here.