Consider a moral argument constructed like this:
1. The case for objective moral values is not as strong as it is for the existence of God.
2. If case for objective moral values is not as strong as it is for the existence of God, and the case is strong enough that one can reasonably believe in the existence of objective moral values, then the case is strong enough that one can reasonably believe in the existence of God.
3. The case is strong enough that one can reasonably believe in the existence of objective moral values.
4. Therefore, the case is strong enough that one can reasonably believe in the existence of God.
I think (1) is defensible, since most arguments for objective moral values tend to come down to moral intuitions. Certainly there are theological intuitions that can be mustered in support of theism, e.g. the sensus divinitatis of Calvin, but there many other sorts of arguments for God for which there is no parallel proof for objective moral values. There are cosmological arguments, arguments from fine-tuning, arguments from consciousness, the argument from desire, other varieties of moral arguments, various ontological arguments, the trademark argument, arguments from miracles, and so on. Now one might say that these arguments, even in their strongest forms, are not successful. Perhaps, but the case for God may still be stronger.
Now someone might object to (2) by saying that even if the case for objective moral values is weaker than the case for God, the strength of the case must be proportionate to the extraordinary nature of the claim. Perhaps the existence of objective good and evil is less extraordinary than the existence of God, so even if the case for God were stronger than for objective moral values and the case for objective moral values were strong enough for someone to reasonably affirm them, one might not have a strong enough case for theism. This depends on what we mean by “strong” or “weak”. If we mean those terms to be some objective quantified assessment, I think this critique would be right. But I mean something else, I think of “strong” or “weak” relative to the conclusion trying to be established. So if I say that the case for God is stronger than the case for objective moral values, I am not saying that the case for objective morals is a “7” while the case for God is a “9”. You could turn around and say that it is reasonable to accept moral values when the case is at a “7”, you need to have a “10” to make a reasonable case for belief in God. Rather, I am saying that if the case is closer to being reasonably established for theism than it is for objective moral values, and the case for objective moral values is rationally defensible, then it must also be for theism.
Finally, I think (3) is going to be a hard one to defend to those who do not already take the existence of objective moral values to be rationally defensible. Atheists who accept (1) are probably going to want to deny (3), and those who accept (3) are going to want to go after (1). Nonetheless, as I said, there are not too many arguments for objective moral values beyond moral intuition. The only other defense is to draw out some of the untoward consequences of denying the existence of objective moral values, e.g. the impossibility of assessing moral progress, the bizarre notion that, at any given moment when one has a moral opinion, that moral opinion is correct at that moment, the impossibility of moral discourse on shared objective values and principles, etc. Now, these are not so much independent arguments, but ways to draw out our moral intuitions more sharply. It is intuitive to me that the success of the civil rights movement was a step towards moral progress. Furthermore, though I would say that I think my moral beliefs are true, I hardly think I am infallible at any given time. I think I can make mistakes about moral assessments, so I think a subjective moral opinion isn’t right just because someone holds it. Finally, I observe moral debate and discourse all around me, on television, in movies, in conversations with friends. It could be that they are engaged in a meaningless exchange, like friends who are engaged in a heated debate over whether Jazz is better than Classical. There is no right answer when it comes to questions of taste, but people can still have debates about them (especially at the pubs). Perhaps that is what is going on in moral debates. Perhaps, but I don’t think so. My intuitions tell me otherwise. I think it is rational to believe in objective morality, and a good percentage of philosophers seem to agree.
If so, I think that we can conclude that the case for God is strong enough to permit a reasonable belief in God. Just a thought.