In the systematic theology 1 class I'm TA'ing, the professor recently made the following observation:
C.S. Lewis posits, in Mere Christianity, that there is a "Moral Law" which all people recognize within themselves, and that this recognition serves as a theistic apologetic. But, to paraphrase Prof. Scalise, "Go to Pioneer Square in Seattle and ask people you meet if they believe in an absolute standard of right and wrong. I think you'll quickly discover that Lewis' argument doesn't work very well with most people."
Now, several students in the class objected to this, responding that Lewis wasn't attempting to prove that there is an absolute standard of right and wrong agreed upon by everyone, but rather that every person has a standard by which they determine morality, and, since each of us has such a "longing" for fairness, justice, etc., Lewis' argument still carries some weight as an apologetic. However, they generally agreed that this longing should not serve as a primary foundation for Christian belief.
This is all correct, as far as it goes. The problem is that we haven't gone far enough with our examination of Lewis' argument. What is Lewis actually hoping to do with the Moral Law hypothesis? It seems fairly clear that he seeks to develop a natural theology of some sort, in the hope that Christianity will be seen as "reasonable" and thereby become more appealing to "rational people." Lewis states:
"The Moral Law, or Law of Human Nature, is... not a mere fancy, for we cannot get rid of the idea, and most of the things we say and think about men would be reduced to nonsense if we did. And it is not simply a statement about how we should like men to behave for our own convenience; for the behavior we call bad or unfair is not necessarily the same as the behavior we find inconvenient, and may even be the opposite. Consequently, this Rule of Right and Wrong, or Law of Human Nature, or whatever you call it, must somehow or other be a real thing – a thing that is really there, not made up by ourselves." (Mere Christianity, p. 30)
Now, Lewis may be right, but he may also be wrong. The Moral Law could be a neurological development that is evolutionarily beneficial for human survival. But, in fact, he could be right about the reality of the Moral Law as independent from humans, and still be wrong! The dilemma is this: Just because Lewis' assertion that everyone has been given a real standard of "fairness" might be true, doesn't mean that it is given to us by God, let alone the Christian God.
In fact, Lewis admits as much. But he continues to pursue his line of reasoning, and that complicates things further. Because, if Lewis relies upon the absolute of the Moral Law as a ground to argue for the reasonableness of faith in Christ, then if that prior foundation is knocked down, the argument falls apart. An atheist might also hold to moral absolutism, that is, agree there are absolute principles of right and wrong. But they would argue that those principles are built into the structures of the natural world. Just as, for example, gravity dictates that objects will be drawn to each other according to a set of principles, human beings are drawn to a particular moral structure based upon a set of principles that dictate what behavior will best benefit human development.
My point is not to argue which of these views of moral absolutism is most reasonable. In fact, I would reject that whole approach, since it is not helpful in ascertaining anything about the Christian God. Instead, I begin with the assumption that, if anything like the Christian God exists, that God surpasses and yet sustains all laws or sets of principles. But God is not a member of a set, or any being/entity that is somehow determinable by a set of principles we posit. In this sense, it is theologically faulty to attempt to derive any knowledge of the Christian God from a prior set of rational or empirical principles.
Now, one might say that, given the possibility of a supernatural explanation for morality (or anything), such explanations can be helpful to bolster the claims of believers, since they offer support for the "general" claims of some sort of God, which might make it easier to subsequently believe in the Christian God. But, the flaw in placing hope in any such explanation is that when/if those explanations are undermined, our faith becomes undermined, because our "specific" claims to Christianity are no longer supported by the general claims of theism.
This danger should always be kept in mind, and it is right to caution Christians against placing their hope in any rational or empirical attempt to provide an explanation for God. These approaches are not entirely useless, but they are not sufficient as a ground for belief. That can only come when one, in the face of the evidence, and in spite of the counter-evidence, commits oneself to faith in Christ and the God of Christianity, not as a subsequent move based upon prior foundational claims, but as THE foundational claim - a claim that is neither rationally or empirically grounded, but grounded in the hope of Christ as the truth that makes sense of all other truths.
Yes, we can claim to have knowledge of an absolute Moral Law, but only in light of the prior claim that our reality is centered by Jesus Christ, who is God. That is our only source of absolute truth. All other truth is objectively valuable, but ultimately uncertain, especially when it comes to questions of faith.