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.
David Hume
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 "old questions"
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.)
Suffering and evil
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.
What suffering?
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.