A redated post.
The is from Mark Nelson's paper Naturalistic Ethics and the Argument from Evil,' Faith and Philosophy, vol. 8, no. 3, 1991.
Nelson argued in this paper that the moral premise of the argument from evil is undermined if the atheist construes that premise in a non-realist way, that is, he does not think that any propositions about what one ought to do can be true.
For reference, here are 1, 2 and 4 to which he refers in the paper.
1) if there were an all-good, all-powerful God, then there would be little or no evil in the world.
2) But there is much evil in the world.
4) If there were an all-good God, he would want there to be little or no evil in the world.
He writes, concerning the possibility of a reductio version of the argument:
Third, while not taking the argument as a reason for atheism itself, the naturalist can still try to offer the argument as an ad hominem argument that anyone who holds the non-relativistic ethical theory that the theist in fact holds should reject theism. That is, even if the naturalist does not believe premises 1 and 2, she can argue that the theist must (or at least does) hold premises 1 and 2, and that these jointly entail 3 (atheism-VR). Since few theists these days deny 2, the real issue is whether the naturalist can show that the theist must, or does, accept 1. In the present context, this boils down to whether the naturalist can show that the theist must, or does, accept 4, and this is a tall order. While some theists accept 4 or ought to, given their other philosophical commitments, it is by no means obvious that all do or even should, since, for theists, the acceptability of 4 depends to some extent on the truth about morality, and even among theists there is considerable disagreement about what this is. In sum, it's not as if the naturalist can point to a set of moral propositions to which all theists must share and say "See! These commit you to 4!" And the theist should be wary of letting her critic pin some definite moral theory on here, since it may be difficult to say what moral theory a world view commits us to, except from a vantage point "inside" it, as it were. Moreover, the theist might regard the ability to handle the problem of evil as a condition of adequacy for any theistic theory of morality. Finally, such an ad hominem argument does not satisfy the conditions for a disproof of the existence of an all-good, all-powerful God.