The 'challenge' I refer to is the challenge to belief in God raised by the problem of suffering. Sam Harris, in his "Letter to a Christian Nation" states it like this.
If God exists, either He can do nothing to stop the most egregious calamities, or He does not care to. God, therefore, is either impotent or evil.
The form of the challenge which most people are familiar with is the famous quote from "Dialogues Concerning Natural Religion" by David Hume.
Epicurus' old questions are yet unanswered. Is he willing to prevent evil, but not able? then he is impotent. Is he able, but not willing? then he is malevolent. Is he both able and willing? whence then is evil?
My personal view is that Hume's prominence is hard to understand: his challenge, as stated, is both imprecise and inaccurate. It is an effective piece of rhetoric, but not a careful argument.
The first point to note is an obvious one: the "old questions" are not unanswered, as Hume must have known: they have been answered by many people and in many ways. Hume may not like the answers; he may not be persuaded by them, but to pretend they do not exist is misleading.
(We should also note in passing that it is now considered unlikely that the "old questions" actually came from Epicurus, but Hume would not have known this.)
More seriously, Hume (in common with many others) ignores the substantial distinction between suffering and evil: they are obviously related, but they are equally obviously not the same.
It seems from the context that Hume cannot be talking about evil as such: he seems to use the term as shorthand for a class of undesirable events.
Evil can only exist in a moral universe. If there is no God, it is hard to see how the universe can be anything other than amoral. Certainly, Hume does not make any attempt to argue the existence of good and evil other than in human terms. Good can be described as what we like, and what tends to produce health and happiness; and evil is the opposite.
But there is no reason to suppose that anything which gets in the way of my pleasure or health is automatically morally wrong. Indeed, if you define them in human terms, they no longer refer to a moral reality.
So if good and evil are merely human categories, then "God is evil" means nothing more than "God does things I don't like". Which is hardly surprising, and not good grounds for concluding that He does not exist.
On the other hand, if good and evil have the meaning we normally associate with them, then we are living in a moral universe and rely on God to tell us what is good and evil.
In short, if the term 'evil' has any meaning, then the use of the word presupposes the existence of the God Whose existence Hume is attempting to disprove.
But if he is really talking about suffering, it is hard (I suspect, impossible) to determine exactly what type of suffering he is referring to and without clarity on this point, we cannot tell what an appropriate response to his challenge might be.
