Rod Dreher has some interests thoughts and link roundups to the idea of Natural Law, by way of explaining how non-religious people need Natural Law to construct a rational foundation for ethics. I think Hume is right on this:
Reason is, and ought only to be the slave of the passions, and can never pretend to any other office than to serve and obey them.
The major problem here is that Christians (and other theists and believers in a fundamental metaphysical system) stand upon their hill, and explain how they have a grounding already. The problem is that they don’t. Saying you have a Ground of Being, and constructing complex narratives and philosophical systems around your assertion, does not a Ground of Being make. If you presuppose that God exists and define God as the Ground of Being, you have a word game. You may believe that this is a rock solid argument, but it is totally ridiculous to someone who rejects the premise of the word game.
This comes up in situations where Christians may object that atheists have no foundation for their morality, how can they justify their morality? But the question can be turned around, because atheists don’t believe Christians have a foundation for their morality either, and they are moral, and make justifications. Christians believe they have a foundation, but a belief in a fundamental fashion does not constitute reality. Concepts are not so concrete. This of course is a philosophical issue. If you are a Platonist of some sort you actually do believe that concepts are concrete! In sum, even the idea that you have a rational Ground of Being is a matter of faith.
The point of this post isn’t to convince anyone, rather, it’s to clear up some misunderstandings which often crop up. This issue is not limited to discussions between atheists and theists. Many atheists themselves cede to Christians the proposition that they have a Ground of Being, while atheism negates that possibility. Where I object is that this shifts the ground rules in a manner Christians are liable to win because the idea of something is transformed into the tacit reality.