One family of arguments among the dozen or so which, taken together, make a strong case for the claim that theism is a better explanation for our experience of the world than is naturalism or, alternatively, that it's more probable that theism is true than that naturalism is true, are the arguments lumped under the heading of The Moral Argument. One version of this argument goes like this:
1. If there is no God then there are no objective moral duties.
2. There are objective moral duties.
3. Therefore, there is a God.
In this argument God is taken to be a transcendent, perfectly good moral authority who is able to hold us accountable. The argument is not a proof since when faced with it the skeptic has a couple of options:
A. He can reject the first premise and argue that even though there's no God there could still be objective moral duties.
B. He could accept the first premise but deny the second premise and thus embrace ethical subjectivism or nihilism.
Of course, if he accepts both premises he's logically bound to accept the conclusion.
The problem is that, as I argue in my novel In the Absence of God (see link at upper right of this page), either option he selects to avoid having to accept the conclusion creates difficulties. If he chooses A then it's incumbent upon him to show where objective moral duties could come from if not from a divine law-giver. Neither society at large nor the cosmos itself is a suitable source of moral value, and any moral duties the skeptic embraces are arbitrary choices.
If he therefore chooses B and embraces some form of subjectivism he has to recognize that his moral choices are simply an arbitrary preference or taste and that he must forfeit the ability to make judgments of anyone else's behavior which are also based on their own preferences which are no more right nor wrong than his are.
This suspension of moral judgment may sound good to someone of a post-modern inclination, but only until one gets down to cases. If our moral duties are all subjectively imposed we can't say that a child molester or rapist, or even the torture of children is "wrong." The most we can say is that these things certainly seem wrong to us, but if they don't seem wrong to the person doing them then in what sense are they really wrong? The idea that these things are not really wrong for the person doing them is extremely difficult to live with consistently. The subjectivist option leads at best to moral egoism, i.e. the view that the right thing for me to do is whatever increases my pleasure and contentment in life, and at worst to moral nihilism, i.e. the view that nothing is really right or wrong in a moral sense.
But, the skeptic will reply, relying on God creates problems for the theist as well. One famous attempt to show that the theist is in no better position than is the skeptic with regard to a foundation for morality first appeared in one of Plato's dialogues (The Euthyphro) in which Plato has Socrates pose the following question to an interlocutor named Euthyphro: "Is something morally good because God commands it or does God command it because it is good?" This is called the Euthyphro Dilemma because it seeks to confront the advocate of the moral argument with two unpalatable choices between which he must choose.
If the theist chooses the first option, that good is whatever God commands, then presumably had God commanded us to be cruel, cruelty would be morally good, a state of affairs which seems to be at the very least counterintuitive.
If the second alternative is chosen, that God commands us to do what is good, then good seems to be independent of God, existing apart from God, and rendering God unnecessary for the existence of good or "right."
Over the next couple of days I'd like to explain why I think the The Euthyphro Dilemma, for all it's popularity, doesn't do the work that some skeptics think it does. More tomorrow.