Among the arguments that philosophers have advanced in support of the claim that there is a God are those which seek to show that God's existence is more probable than his non-existence. Since, therefore, we should always believe what is more probable than what is less probable, belief in God is rationally warranted. One class of arguments of this type which doesn't receive as much exposure as it deserves, at least in my opinion, are arguments based on the existence of objective morality.
The following short video illustrates one version of this type of argument:
In other words,
1. If there is no God then there are no objective moral duties (OMD).
2. There are objective moral duties.
3. Therefore, there is a God.
Of course, this is not a proof that God exists because a skeptic could certainly deny either premise 1. or premise 2., but there's a steep cost for so doing.
For example, if one chooses to deny 1. and claim that OMD can exist without there being a God, then it's incumbent upon the denier to explain how anyone, in the absence of a transcendent moral authority, could have an objective duty to perform any act at all. Who, or what, can impose such duties upon us? Where do they come from? The only adequate source of moral obligation must be both personal and morally qualified, and there's nothing other than God that can fulfill those requirements.
On the other hand, if one chooses to deny 2. then one must either embrace some form of moral subjectivism or acquiesce to moral nihilism, i.e. the denial of the existence of any moral duties at all. Either option entails that one has no grounds for making any moral judgment regarding the conduct of others. Subjectivism may seem more attractive than nihilism, but it leads to it in any case since it puts one on a slippery slope to to the view that there really is no right or wrong, just different feelings and opinions held by different people.
Thus, if a person believes that each of us has a duty to refrain from things like molesting children, abusing women, torturing animals, rape, murder, robbery, slavery, etc., and if one believes that those who do these things are morally guilty of having done a great wrong, the person is neither a subjectivist nor a nihilist. In fact, that person is tacitly acknowledging that there are moral principles which are objective and binding on all of us.
But how do we account for the existence of OMD? Surely, the existence of such duties is more probable on the hypothesis that God exists than on the hypothesis that he doesn't. Indeed, if naturalism is true OMD are inexplicable.
