Belief in God can certainly be irrational. We might ‘believe in’ God because we like the idea of God’s existence or because religious behavior has financial or social benefits. 
In these cases, ‘believing in’ God is a bit like ‘believing in’ your city’s professional baseball team; it has little to do with an intellectual belief and much more to do with your feelings. But if belief in God can be irrational, does it follow that belief in God is always irrational? I don’t think so.
A belief is rational if it is held on the basis of reason and logic, rather than on the basis of emotion or pragmatism. If we can demonstrate that belief in God can be the consequence of a logical argument with justifiable premises, then it follows that belief in God can be rational. I’d like to focus on one such argument in this essay: the moral argument for God’s existence. Many prominent atheist thinkers endorse both premises of this argument independently. If we conclude that these atheists present good, justifiable reasons for believing these two premises independently, then it seems rational to hold them simultaneously and to conclude that God exists. If so, then belief in God’s existence can be rational.
The most common form of the moral argument runs as follows:
Premise 1. If God does not exist, then objective moral facts do not exist
Premise 2. Objective moral facts do exist
Therefore,
Conclusion. God exists
By ‘God’, I mean a perfectly good, personal being whose commands are binding on all human beings. By ‘moral facts’, I mean objective statements about which values and actions are good and evil or right and wrong. For the purposes of this essay, I am not interested in whether either of these premises is true (interested readers can see a discussion of this issue here). Instead, I merely want to ask whether both of these premises can be held for rational reasons on the basis of arguments rather than on the basis of emotion. I believe that they can.
Is premise 1 rational?
Atheist philosopher J.L. Mackie writes in The Miracle of Theism that “[m]oral properties constitute so odd a cluster of properties and relations that they are most unlikely to have arisen in the ordinary course of events without an all-powerful god to create them.” His opinion was shared by French atheist Jean-Paul Sartre, who writes in Existentialism is a Humanism that “God does not exist [and] there disappears with Him all possibility of finding values in an intelligible heaven… Everything is indeed permitted if God does not exist.” Similar sentiments were expressed by Bertrand Russell and Friedrich Nietzsche, who both insisted that the nonexistence of God entailed that moral facts were likewise nonexistent.
The arguments of atheistic philosophers who affirm premise 1 of the moral argument are somewhat varied, but they share several features. First, they argue that objective moral values like ‘love’ or ‘justice’ seem to have no place in a reductively naturalistic universe. Physical facts can usually be observed and measured. But purported moral facts don’t seem to refer to physical objects or to things which can be physically observed; how can you measure ‘justice’ in a laboratory? So if -like many atheists today- we insist that only empirically measurable, natural entities exist, how can we simultaneously hold that moral facts exist? Second, on what basis do we insist that human beings have some intrinsic value that we are obligated to respect? If there is a God and human beings are made in His image, then He can impart value to them. But if human beings are merely complex, self-contained biochemical reactions, what makes them objectively more valuable than any other complex, self-contained biochemical reaction – like the one I carry out in testtube in my biology class and then pour down the sink? Finally, why are we morally obligated to do anything at all, whether to respect the environment or love other human beings or seek the truth? We may perform certain actions out of self-interest or as a consequence of our personal preferences or because of instinctive biological urges. But if there is no transcendent moral lawgiver who obligates us to act in a certain way, then how can we insist that there is a transcendent, objective moral law?
It is on the basis of these and other objections that the aforementioned atheist philosophers concluded that premise 1 of the moral argument is true: if God does not exist, then objective moral facts do not exist. It is possible that they are wrong in holding this belief. But it is difficult to insist that they are irrational in holding this belief, since it is based on philosophical argumentation rather than on emotion or pragmatism.
Is premise 2 rational?
The second premise of the moral argument enjoys even more support from atheists. For example, a recent survey of professional philosophers found that 56% were moral realists who believe that objective moral facts do exist. Notable atheists who insist that objective moral facts exist include Thomas Nagel, Michael Martin, Kai Nielsen, Louise Antony and Sam Harris.
The main argument used to support the existence of moral facts is the observation that moral realism is a deeply intuitive position. Normal human beings seem immediately aware of a moral realm: that there exist objective categories of good and evil and right and wrong. Even professed moral relativists find it difficult if not impossible to remain consistent with regard to their moral relativism. While many atheists might be uncomfortable appealing to `intuition’ to support a philosophical view, there are many extremely basic beliefs which we hold for the same reason. For example, our belief that the external world actually exists (i.e. we are not living in the Matrix) or our belief that other people have first-person, subjective experiences of pain and pleasure (i.e. they are not unconscious zombies) are both based primarily on intuition. No one can prove that these intuitions are true, but it seems ridiculous to insist that they are false. In the same way, moral realists would argue that the denial of such a basic experience as the existence of moral facts will always be less plausible than the affirmation that such facts exist. Nielsen summarized his position in this way:
It is more reasonable to believe such elemental things (as wife-beating and child abuse) to be evil than to believe any sceptical theory that tells us we cannot know or reasonably believe any of these things to be evil… I firmly believe that this is bedrock and right and that anyone who does not believe it cannot have probed deeply enough into the grounds of his moral beliefs. – Kai Nielsen, Ethics without God, 1990.
Again, atheists who are moral realists could be wrong. But it seems difficult to insist that they are irrational in their acceptance of premise 2 of the moral argument, any more than we could insist that someone is irrational to believe that the external universe exists or that other people have minds.
But if both premise 1 and premise 2 of the moral argument can be held independently for rational reasons, what is to stop a person from holding both premises of the moral argument simultaneously and consequently arriving at the logical conclusion that God exists? Nothing, it would seem. The only objection we might raise is that it is possible to have some overriding, independent reason to believe that God does not exist (in philosophical parlance, such a belief is known as a ‘defeater’). However, even if we grant that some people have such defeaters for theism, many people do not. In my experience, few people who are not avowed atheists would claim that they have sufficient evidence to believe that God does not exist. Even passionate atheists like Richard Dawkins would not claim to be certain that God does not exist! So a typical agnostic who hears the moral argument can rationally accept both premises and can rationally conclude that God exists.
Conclusion
Belief in God can based on emotion, but it can also be based on reason. In particular, it can be based on philosophical arguments for God’s existence such as the moral argument that I discussed above. Of course, this essay does not address or even attempt to address whether the moral argument is sound. I think it is. But if you identify as a skeptic, an agnostic, or an atheist, my more modest goal is to demonstrate that belief in God need not be irrational.
Should that recognition be the end of our deliberation about God? Certainly not! If God does exist and is anything like the God described by Christianity, then He is more important than anything else in our lives. So I would encourage you to think deeply about these questions and not brush them aside. While admitting the rationality of theism is not a place to stop, it might be a place to start.
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