*Note: I'm not very happy with the way this post turned out. I reserve the right to make edits later, particularly if it comes out in the comments that I was unclear on something*
Many Christians (and indeed, I myself) are fond of pointing out that Atheists seem to lack any solid metaphysical grounding for the moral intuitions by which they live their lives. Many Christians also take it as a given that Christianity solves this problem. But I'm not so sure.
The typical Christian claim goes something like this: there is an omnipotent, omnipresent, omni-benevolent God. This God is the arbiter of morality- he is both its source and it's ultimate judge. That which we call "good" could more accurately be termed "of the character of God", and that which we call "bad" is actually that which is "separate from God".
But it seems to be the case that morality in this framework is just as arbitrary as in the Atheist's. It arises purely based on the character of God. That is to say, if God's character had been different, morality itself would have been different. But in reality, there seems to me a clear difference between what we call morally good and morally bad behaviors. Morally good things are things that fundamentally benefit people- make them more comfortable, happier, more secure, more free. Things that we call morally bad are things that fundamentally hurt people- cause them discomfort, pain, anguish, uncertainty, and fear. Morality, in practice, is far from arbitrary.
Now the argument can certainly be made that morality is not arbitrary precisely because God's character is not arbitrary- we only consider morality "fixed" because God's character is fixed. But I don't think this solves our problem, if only because we can conceive of a God with a different character than omni-benevolence (certainly the ancient Greeks did). Just because a being is omnipotent, it does not follow that his moral character is omni-benevolent- even if we are somehow justified in making this claim on God's character, we're still constraining God's character to some external stanard of the moral law.
Put it another way- "Omni-benevolent" is a descriptive word that we apply to God after his character is revealed. If omnibenevolent simply meant "whatever God is", we wouldn't need to bother calling God omni-benevolent. Even if we make the claim that omnibenevolence is "part of God's nature", that's no help here. After all, cheerfulness is part of Steve's nature. It may be the case that Steve is perpetually and unchangeably cheerful, but the definition of cheerful does not derive from Steve. We don't look at Bill (also cheerful) and say "you're so Steve". Steve and Bill both share the "Cheerful" trait, and are even defined in relation to that trait, but that trait does not derive its meaning from them. "Cheerful" would still be a thing, even if they didn't happen to be cheerful. Much the same, we may claim God is omni-benevolent, but that is a very different thing than saying omni-benevolence derives its meaning from the character of God. If those two are the same, then talking about an evil (or even just imperfect) God wouldn't just be nonsensical, it would be syntactical jibberish.
Put it yet another way- we like to call God omnipotent. But nobody makes the claim that the definition of power is "whatever it is that God has infinite amounts of". Power is a thing, whether or not there is a God to have it. We can say that God is by his very nature all powerful- and that may well be true- but it does not invalidate the concept of power in a framework that doesn't include God.
I think the Evil God problem illustrates that "God", while perhaps being necessarily "good" by virtue of his omniscient nature (as Leah contends in that link), is still a logically seperable concept from "good" (even if God is necessarily good in Leah's framework, then He is being constrained by good, not the other way around). But I don't think we even need to appeal to that level of argument to get this point across to a Christian audience (since I suspect the evil God idea will strike many as fundamentally absurd). Instead, let's look at the story of Abraham. God directed Abraham to kill his only child in cold blood. This is pretty clearly a morally evil action to take. If you hear a news story about a women who kills her children because she thinks God told her too, you don't think "good for her, believing her faith so strongly", and you don't for a second question whether or not she was actually doing as she was told by God.
When I've disussed this question with Christians, I've never heard any of them say that killing Issaac wouldn't have been wrong because anything God says is by definition right. Instead, they either point to the fact that God didn't actually make Abraham do it, or suggest the possibility of far-reaching consequences we're not capable of grasping (i.e. killing baby Hitler). In either case, we are saying that God simply would not order a senseless killing without some good reason. This is not a claim that God is morality, but rather that God is always right about morality- and since God knows more than us, and because he has the character of omni-benevolence, we are required to trust him completely. We are admitting here that the standard of morality is external to God- some things are right and some things are wrong, and God's character fits entirely into one of those categories. And again, even if his character necessarily fits into one of them, that is God being constrained by morality, not the other way around.
It seems to me that the Christian is on no more solid footing here than the Atheist. Both are appealing to moral standards that have roughly the same metaphysical grounding. But here's the thing- I don't ultimately see this as a problem. It seems pretty straightforward to me, once you've arrived at a position of valuing human life and the human experience, to arrive at a morality that says "people matter, and what's good for people is morally good and what's bad for people is morally bad". The question that Christianity tries to answer (and where Atheism fails for me) is why we should care about morality in the first place? Coming up with morality is pretty easy, except in the most extreme of edge cases; convincing me why I should care about some arbitrary electrical signals passing through a semi-randomly arranged mass of protons and electrons is a much tougher deal.
All the arguments I've read for secular morality (including the one I'm in the middle of from Why I Believed) point to rational self-interest as our reason for being moral. This is where they all lose me. Certainly rational self-interest is a tremendous motivator (the only thing I learned from my college economics class was the following dictum: "Incentives work"), but that's not why I want to be good (or rather, not why I want to want to be good). I want to be good because it's the right thing to do- not because of potential divine judgement, and not because it will benefit me in the end. Certainly my desire- and a reasonable basis for morality- are explicable in the secular vision. They're just totally, completely unsatisfying to me. It seems to be the case that once I recognize that morality is just a hard-wiring of my brain to prefer things beneficial to the group, my optimal strategy is to actively override my moral intuitions. I should be good when it suits me, in case others are watching, but in the case where I'm sure nobody will notice, there's absolutely no motivation for not doing what benefits me the most. I would certainly never donate anonymously, sacrifice myself for another, or anything else that my corrupted-hardware-brain might try to convince me to do out of some misplaced burden of evolutionary psychology. Wanting it to not be true is not an indication of it actually not being true- but man is that a depressing reality