“My argument against God was that the universe seemed so cruel and unjust. But how had I got this idea of just and unjust? ... Of course I could have given up my idea of justice by saying it was nothing but a private idea of my own. But if I did that, then my argument against God collapsed too – for the argument depended on saying that the world was really unjust, not simply that it did not happen to please my fancies.Thus in the very act of trying to prove that God did not exist – in other words, that the whole of reality was senseless – I found I was forced to assume that one part of reality – namely my idea of justice – was full of sense.”
The main assumption here is that objective moral values – and therefore the existence of good and evil, just and unjust – cannot exist apart from God. And since the atheist does not believe in God, she cannot consistently claim that there is evil or injustice in the world.
The idea that objective moral values are dependent upon God is something that can easily be disputed, and many have done so. But there is another assumption here that isn't quite as obvious. This is the idea that someone raising the problem of evil must, if she is to be consistent, believe that there is such a thing as objective evil. As Lewis says, if good and bad are just ideas in your head, then the argument against God collapses. This assumption, however, is wrong, for at least two reasons.
To begin with, anyone raising the argument from evil may claim that she is presenting a problem that is internal to the theistic worldview. That is, she may say that, though she does not believe in the existence of evil herself, the theist does believe in it, and the problem is that the theist cannot account for that supposed evil. That this is correct can be seen from the fact that the question of evil is something often discussed by the religious amongst themselves. This shows that given their worldview, there is at least a potential problem here.
The second reason the above is wrong is more serious, however. Those who agree with Lewis usually maintain that one who does not believe in objective values cannot regard anything as good or bad. For example, Douglas Wilson, the pastor who debated Christopher Hitchens in the movie Collision, said he could understand an atheist who was a nihilist, but not one like Hitchens who was full of moral indignation over the injustices he perceived. Similarly, apologist Frank Turek claimed that:
“Either evil exists or it doesn't. If it doesn't exist, then atheists should stop complaining about the 'evil' religious people have done because they haven't really done any.”
But the fact is that there is a third alternative between, on the one hand, believing in objective values and, on the other, holding that nothing can be regarded as good or bad. Wilson and Turek are simply mistaken. If I don't believe in objective evil – because I don't believe in objective values – it does not follow that I don't regard anything as evil, nor that I should stop complaining when something that I regard as evil is done. Obviously, if I believe that values are subjective, I still believe in values – namely, subjective ones!
I have moral views which say that (for instance) torturing sentient beings for fun is always wrong. I may not believe that there is a fact, discoverable by science or by philosophical analysis, that corresponds to the statement “torturing sentient beings for fun is always wrong,” but that doesn't mean I wouldn't oppose such an action. In fact, I would oppose it with every fiber of my being. And I would certainly call it evil.
Nor is this merely my subjective opinion. There is actually a great deal of intersubjective agreement on such issues. The vast majority of us are opposed to murder and rape, for example. It's this kind of intersubjective agreement that allows us as individuals to intelligibly communicate with one another regarding moral questions. Thus, when someone claims that God is good, they presumably intend to say, among other things, that God does not approve of murder, rape, or torturing someone for fun. It follows that the claim “God is good” can be meaningfully discussed on a subjectivist understanding of morality.
Someone who claims that God is good is presumably claiming that God doesn't want there to be such evils in the world as diseases that kill millions every year. After all, anyone who desired such a thing wouldn't normally be called good. And it is because there are such evils – things that the great majority of us regard negatively – that there is a problem for theism. The objectivity of values has nothing to do with it.
The Lewis quote is from Mere Christianity.
Turek's comes from "Why Evil Disproves Atheism,"