Plantinga has argued publicly that perhaps one of the reasons God has for permitting evils like the holocaust, or the social experiment of Marxist communism, or any other evils you might think of, is because of the intrinsic goodness of something like atonement. The idea being reflected in St. Augustine, and in the Mass, that, as the saying goes, Felix Culpa. When God sits back before the creation of the world and tries to decide what kind of world to make, obviously he wants a good world, but (we might ask) in light of what exactly does a world qualify as good? What kind of features add to a world’s maximal or ideal goodness. Of course, strictly speaking we might think that creaturely pleasure should be a good, but first this feature of a world (like many others) cannot be maximal, and isn’t necessarily an ideal which takes precedence over other more significant ideals which might make it dispensable.
Maybe one of the great goods, Plantinga suggests, is the good of an atonement, an incarnation, a redemption, and, in short, a world in which God actualizes something like the Christian story (Plantinga doesn’t quite go so far as to make this claim, but I’m extrapolating, I think, fairly). Such worlds would either need to contain evil, or else at least need to be able in principle to contain evil. Moreover, no evil is in principle too evil to be permitted so long as that evil does or can in principle give opportunity for God to illustrate his love that much more. As Jesus of Nazareth once said:
‘A certain creditor had two debtors; one owed five hundred denarii, and the other fifty. When they could not pay, he cancelled the debts for both of them. Now which of them will love him more?’ Simon answered, ‘I suppose the one for whom he cancelled the greater debt.’ And Jesus said to him, ‘You have judged rightly.’
So long as no evil is too evil to be forgiven/redeemed, any evil, however horrendous, can in principle be an occasion for a greater illustration of God’s love in redemption.
Thus, on this apology, Christianity comes with a ready-made defeater for the problem of evil. Notice that this may be adapted appropriately to work for the moral, natural, metaphysical and epistemic problems of evil (where it may not work for the metaphysical problem of evil, other solutions offer themselves pretty readily).
Perhaps Theism in general does have a genuine problem of evil, to which Christianity offers a solution, but Christian Theism in particular seems invincible against the problem(s) of evil. On Christianity one isn’t or shouldn’t be surprised to find all the evils we find, or even the extent of evils we find, in the world. Now, of course, if evil alone were a great-making property of worlds we could imagine a worse (or, to nerd out and quote an oft cited meme, ‘worse, or better?‘) world. However, the point is that the world is supposed to have evil for the sake of a greater good which could not be obtained without evil in principle. Theism in general can at best speculate on what the greater good might be, but on Christianity the answer is given: the world is supposed to exemplify atonement and redemption.
Moreover, Christianity is not the kind of thing which a Theist would be likely to think up as a response to the problem of evil (historically Christianity does not arise as a response to the wider problems of evil), and so it certainly isn’t gerrymandered to be a response to the problem of evil. It’s answer is surprisingly not ad hoc.