As a Christian, I cannot imagine any answer to the question of evil likely to satisfy an unbeliever; I can note, though, that—for all its urgency—Voltaire’s version of the question is not in any proper sense “theological.” The God of Voltaire’s poem is a particular kind of “deist” God, who has shaped and ordered the world just as it now is, in accord with his exact intentions, and who presides over all its eventualities austerely attentive to a precise equilibrium between felicity and morality. Not that reckless Christians have not occasionally spoken in such terms; but this is not the Christian God.
The Christian understanding of evil has always been more radical and fantastic than that of any theodicist; for it denies from the outset that suffering, death and evil have any ultimate meaning at all. Perhaps no doctrine is more insufferably fabulous to non-Christians than the claim that we exist in the long melancholy aftermath of a primordial catastrophe, that this is a broken and wounded world, that cosmic time is the shadow of true time, and that the universe languishes in bondage to “powers” and “principalities”—spiritual and terrestrial—alien to God. In the Gospel of John, especially, the incarnate God enters a world at once his own and yet hostile to him—“He was in the world, and the world was made by him, and the world knew him not”—and his appearance within “this cosmos” is both an act of judgment and a rescue of the beauties of creation from the torments of fallen nature.