Here is an observation by Victor Reppert:

One interesting point about many ethical philosophies is that while they make no reference to a theistic God, they do seem to be grounded in metaphysics, and the kind of metaphysics at work is one that a modern naturalist would have trouble accepting. Plato, Aristotle, and the Stoics are prime examples. Plato's moral philosophy is based on the Form of the Good, which shares certain characteristics with the theistic God, and which can be known through a process of recollection where we recollect what we were aware of in a pre-existence. Aristotle is based on the idea of an inherent purpose for human life, and Stoic ethics is a response to Stoic metaphysics. No one seems to be suggesting that ethics will be all just the same regardless of metaphysics. Even if a personal God isn't required for ethics, doesn't it seem plausible that at the very least some sort of metaphysics is required that most naturalists today would have a hard time accepting. Is it reasonable to reject what Kant called a metaphysics of morals?

There's a good discussion going on in the comments section of Reppert's post. I think Reppert is right on this. The old apologetic claim that you can't have morals without God is not exactly right. The naturalist does indeed have some philosophical options for believing in a true, objective moral system in the absence of the Lawgiver of traditional theism. He can be a Platonist, for example, and there are other cognitivist ethical theories that posit some objective ground of moral facts that fit nicely with naturalism broadly defined (see this book, which I recently started but have unfortunately had to set aside, for a nice roundup of the current metaethical scene). But Reppert's point is that the available options don't fit too well with "modern" naturalism, by which I assume he means the form of naturalism that is dominant in the sciences, which is very often physicalist in character. When you start talking about abstract "facts" on a naturalistic worldview - facts like, "A square has four right angles," and, "It is wrong to toss little Jimmy into the well for laughs" - you have to do some wiggling to claim that the facts represent objective non-mental realities. A naturalist can't put a square, for example, in a test tube and study it. He can't do it with the moral fact proscribing the tossing of little Jimmy into the well either. So if he is going to say that these things are real in the same way that the chair I am sitting on is real, he has to monkey with his metaphysics so that immaterial realities can co-exist with the material realities.

Here's an interesting metaphysical exercise: imagine a possible world W where nothing exists, and by "nothing exists" I just mean that in W the standard furniture of the universe is absent: physical things like gravity, planets, and human bodies and personal things like you, me, and God. Now, as Christians we will say that no world can exist without God, since He is the necessary Being who grounds all other being. This is right, but I'm setting it aside for this exercise. Our question is this: would it be right to say of this world that "nothing exists" as I have just done?

Maybe. But to answer the question we need to tackle the sticky issue of the ontological status of abstract entities such as numbers, propositions, and certain types of properties. We are tempted to say of W that it is truly an empty world, that is, that it is right to say of it that "nothing exists." But consider the number seven; would it exist in W? Certainly there would be no entity in W that would instantiate what we might call a seven-related property, and a seven-related property is a property like having seven parts. In W there wouldn't be, say, a mousetrap made up of seven parts, and thus there wouldn't be anything exemplifying the property of having seven parts. Further, since nothing physical like a mousetrap exists in W, then no seven-related property is instantiated at all. But again, would the number seven itself exist?

Bill Vallicella has an interesting post on why, in the end, metaphysics is more important than logic. He deals with Fred Sommers' turn away from the former to the latter because he thought it was somehow a superior field of philosophical inquiry. But I think the Maverick is right here:

At the end of the day, how important is it to prove that the inference embedded in 'Some girl is loved by every boy so every boy loves a girl' really is capturable, pace the dogmatic partisans of modern predicate logic, by a refurbished traditional term logic? ... As one draws one's last breath, which is more salutary: to be worried about a silly bagatelle such as the one just mentioned, or to be contemplating God and the soul?

This reminds me of something the late Ron Nash said in one of his introductory textbooks. He was discussing the analytic turn in twentieth century philosophy and the suspicion analytic philosophers often held for "system building." He writes:

During my master's and doctor's studies in philosophy, I took many courses from such analytic philosophers. I remember spending one semester examining a single sentence from the writings of David Hume. I spent another semester exploring the two-word expression "I can." I look back with admiration at the creativity of the professors, even though I remember many days in which I felt certain there were better ways to spend my time.

I understand the sentiment here. Neither Nash nor Vallicella are against logical or linguistic analysis (as one look at the Maverick's blog can tell you). They just (rightly, in my opinion) think that there is no good reason to prefer logical questions over metaphysical ones. Linguistic and logical analysis may seem to be more fruitful and accurate methods of inquiry, but they really aren't, and metaphysics - which is more likely to deal with traditional religious questions such as sin, salvation, and eternal life - is actually more important. We are reminded here of William James' notion that religious questions present us with forced options. We may not be certain about the answer to the questions, but their (potentially eternal) importance forces us to make a decision one way or the other.

I think this point is important for Christian philosophers. I see a tendency for some young Christians who want to do philosophy start out full of energy and vigor for being Christian philosophers. That is, they want to do philosophy to the glory of God and to provide rational support for the faith. But then they do their M.A. or their Ph.D. at a state school and their fire for the faith gets dimmer and dimmer, until in the end they may still call themselves a Christian philosopher, but really they're just a regular philosopher who happens to attend Sunday services. They busy themselves with current problems and issues in analytic philosophy, most of which are (1) not concerned with or (2) downright dismissive of religious issues. And this happens, I think, because of what Vallicella was talking about: they steep themselves in the received academic wisdom, which - since the abandonment of Hegelianism by Russell and others around the turn of the century - sees logical and linguistic analysis as superior to religion and metaphysics.

And I think this is tragic. These Christian philosophers aren't apostasizing by any means (although some of them do apostasize, and others get dangerously close), they're just becoming useless for the kingdom. They don't want to risk their academic necks by forcefully arguing for theism or Christianity, and so they stay in their safe philosophical ghettos. But I think they have forgotten an important notion that comes from (of all places) the writings of Karl Marx: the point is not to understand the world, but to change it. Now, I think we Christians can appreciate the point here while promoting a very different type of change for the world than foisting the silly economic system of Marxism upon it. As Christian philosophers, we should seek both to understand the world by doing philosophy and to change it by advancing the kingdom of God. To abandon the latter for supposedly good philosophical reasons will not excuse us on the day of judgment. God is interested in the soundness of syllogisms, but He's more interested in the salvation of souls.