From Alvin Plantinga's spiritual autobiography (pp. 29-30 of the PDF file):
It is indeed true that suffering and evil can occasion spiritual perplexity and discouragement; and of all the anti-theistic arguments, only the argument from evil deserves to be taken really seriously. But I also believe, paradoxically enough, that there is a theistic argument from evil; and it is at least as strong as the antitheistic argument from evil. (Here I can only sketch the argument and leave it at an intuitive level.) What is so deeply disturbing about horrifying kinds of evil? The most appalling kinds of evil involve human cruelty and wickedness: Stalin and Pol Pot, Hitler and his henchmen, and the thousands of small vignettes of evil that make up such a whole. What is genuinely
abhorrent is the callousness and perversion and cruelty of the concentration camp guard, taking pleasure in the sufferings of others; what is really odious is taking advantage of one's position of trust (as a parent or counsellor, perhaps) in order to betray and corrupt
someone: what is genuinely appalling, in other words, is not really human suffering as such so much as human wickedness. This wickedness
strikes us as deeply perverse, wholly wrong, warranting not just quarantine and the attempt to overcome it, but blame and punishment.
But could there really be any such thing as horrifying wickedness if naturalism were true? I don't see how. A naturalistic way of looking at the world, so it seems to me, has no place for genuine moral obligation of any sort; a fortiori, then, it has no place for such a category as horrifying wickedness. It is hard enough, from a naturalistic perspective, to see how it could be that we human beings can be so related to propositions (contents) that we believe them; and harder yet, as I said above, to explain how that content could enter into a causal explanation of someone's actions. But these difficulties are as nothing compared with seeing how, in a naturalistic universe, there could be such a thing as genuine and appalling wickedness. There can be such a thing only if there is a way rational creatures are supposed to live, obliged to live; and the force of that normativity--its strength, so to speak--is such that the appalling and horrifying nature of genuine wickedness is its inverse. But naturalism cannot make room for that kind of normativity;
that requires a divine lawgiver, one whose very nature it is to abhor wickedness. Naturalism can perhaps accommodate foolishness and irrationality, acting contrary to what are or what you take to be your own interests; it can't accommodate appalling wickedness. Accordingly, if you think there really is such a thing as horrifying wickedness (that our sense that there is, is not a mere illusion of some sort), and if you also think the main options are theism and naturalism, then you have a powerful theistic argument from evil.
Note that Plantinga uses the term 'naturalism' in its philosophical sense: the metaphysical view that the universe comprises only material objects acting and being acted upon in accordance with natural law. A naturalistic stance rejects the reality of any supernatural entities or laws (God, angels, non-material human existence).