A recent New Yorker profile of the philosopher Derek Parfit mentioned that the late Bernard Williams once dismissed the ideal of a universally compelling moral code as (I paraphrase) "something you use on the men who come to take you away." Indeed, implied in the "problem of evil" is the conviction (or fantasy perhaps) that if we could only find the right combination for the great moral mystery vault, the ponderous door of error would swing open, releasing a radiance that would burn away the scales from the eyes of the benighted.
I imagine that some religious believers have a similar feeling that if they could only depict or praise God rightly, his existence and glory would be as plain to everyone else as they are to them. The holy grail of thought is the proposition (or grand scheme of propositions) that is as self-evident as 2 + 2 = 4 but as transcendent and as life-changing as the existence of God. That is the constructed idea(l) that we imagine would stop the bad men in their tracks and bring them to their knees. If God does not exist, then it will be necessary to invent (it)--this is the project that is at least implicit in non-relativistic philosophy. As Wallace Stevens wrote, "One day they will get it right at the Sorbonne."
I once read a review by Helen Vendler in which she claimed that the role of the critic is not only (or even primarily) to explain or to justify, but also to celebrate. Similarly, I think that for anyone who reflects seriously about the moral life, explanation and justification go only so far, beyond which point one can only aspire to praise and embody one's views. The barbarians who burn down the monastery are unfazed by the crucifix; likewise, no secular moral system achieves the potency of a talisman. To accept this is also to accept a troubling existential diversity in human nature--other people see the great questions in the same way that I do, except when they don't do so at all. Perhaps the Tower of Babel is the central metaphor for humanity, making us the most atypical species. There is a strain in philosophy that seeks to tear down the tower in favor of a second Garden of Eden, done rightly this time.
The problem is that many men (most of them, alas, have been men) have been sure that they beheld the Truth, and terrible things have been done in the name of Truth. The point is to religiously (in the generic sense) embrace a system of meaning while avoiding clinical or moral insanity. Just as Satanism may be an internally consistent religion, so may there be functioning philosophies of evil (National Socialism, al Qaeda, etc.). We denounce them not because they have no justification (they do have their internal justifications), but because we find them pernicious and repugnant. Our grounds for doing so may be ultimately contingent on the creatures that we evolved to be, but that is the best we can do--we can never escape history by inventing ourselves de novo. By and large, we also happen contingently to find the blues and golds of sea, sky, and sun to be gratifying, and we can only be grateful that we do so. The truth is not given in any simplistic way, but there is also no truth that does not derive, in some fantastically complicated way and filtered through many generations of human consciousness, from our origin.