Most people don't believe in God. At least not really.
I'm speaking primarily of people who claim to believe in God. My assertion isn't that no one really believes in God. It's merely that far fewer people than you might think really do believe in God.
This assertion does not itself reflect any view on whether God actually exists. It is instead about whether people believe that God exists. The philosopher Georges Rey aptly refers to the claim that they don't as "meta-atheism."
My inclination towards meta-atheism first developed as I studied, for my recent book The God Question: What Famous Thinkers from Plato to Dawkins Have Said About the Divine, the history of theodicy -- that is, the efforts by various thinkers over time to explain how the all-powerful, all-knowing, and all-good God might allow all the horrible imperfections and evils in the world. Two things especially struck me as I worked my way through these writings. First, this so-called "problem of evil," which atheists regularly raise against religious belief, is not as instantly destructive of theism as many atheists think: many sophisticated, insightful, and profound things can justifiably be said in response to this problem. But second, more importantly, the more sophisticated, insightful, and profound the responses to the problem become, the more, it strikes me, it is impossible for anyone to genuinely believe them.
For just one notorious example, the 17th-century thinker Leibniz argued that God, by His nature, would create the best of all possible worlds; and since sometimes certain evils are necessary in order to bring about greater goods, even the best of all possible worlds, overall, might have to include various amounts of evil. When you follow this idea into its details, you will discover that, strictly speaking, it may work: on some philosophical level you may be able to reconcile the existence of God and all the evils in our world.
Except for one thing: it seems literally incredible, beyond believable, that this world -- with its history of wars, diseases, natural disasters, deaths of innocent children -- is the best of all possible ones. Voltaire's great novel Candide found Leibniz the best of all possible targets for lampooning, since almost no one, not even those inclined towards belief in God, could take that idea really seriously.
Meta-atheism is also supported by more contemporary things than dusty old philosophy books.
Hypocrisy is of course not limited to believers, but one must be struck by some of the more flagrant cases of religious hypocrisy regularly in the news. Television preachers' criminal activities, priests engaged in pedophilia, evangelicals publicly condemning homosexuality while privately practicing it, family-values legislators conducting extramarital affairs, even religious teens promoting abstinence while sexing up: there is no shortage of alleged believers violating the tenets they claim to believe.
One common response, of course, is to cite our weakness of will, or sinful nature. But ask yourself: if you really believed in an all-powerful God who condemned that behavior, who condemned (for eternity) individuals who engaged in that behavior, could you really, even for a second, engage in that behavior? (Compare: if you really believed that the bridge you were about to cross was going to collapse, would you even chance driving over it?)
Actions do speak louder than words: they reflect what we really believe. And most religious believers regularly engage in actions inconsistent with genuine religious belief. So they must not really believe after all.
There are many other, more mundane, examples. Even believers may not believe that this is the best of all possible worlds, but surely they believe that everything that occurs does so because it is God's will that it occur. But then why, exactly, should anyone get upset when anything happens, since everything that happens does so because the all-knowing and all-good God wills that it occurs? Or more painfully, people grieve when they lose their loved ones, especially children. But if you really believe in God, what is there to grieve about? This world, the actual world we inhabit, is far inferior to the afterlife, to heaven, to being with God. Certainly a child who dies young, before ever having the opportunity to be morally culpable for anything, would merit the more positive final disposition rather than the more negative. But then her death would be an occasion for rejoicing, on her behalf, not an occasion for grieving on our own.
This brief case for meta-atheism doesn't address all forms of "belief," of course. These days, lots of people will say (for example) that they believe in "something greater," even if they don't sign on to any specific set of institutional religious beliefs. But then again, if you don't believe in a "something greater" with the specific properties God allegedly has, then it isn't God you believe in either.
So most people do not believe in God, even if they claim -- not merely to others, but even to themselves -- that they do.