As the people said to Moses in the book of Exodus when he was instructing them in faith, “na’aseh v’nishma”: We will do and then we will understand. That change in belief might come from experiencing benefits you didn’t expect, but it also reveals a basic truth about the mind. As much psychological research over the past sixty years has demonstrated, people’s beliefs will change to match their actions.

Should you believe in God? For many people, belief is a matter of faith. But let’s say you didn’t grow up with a religion, or you fell away from religion, and now are wondering whether that was the right choice. Is there a way to approach the question with reasoned analysis, as you would any other major decision? A rational way to optimize your costs and benefits?

It might seem a strange question. But Blaise Pascal offered a solution to it almost four hundred years ago, using what scientists now call decision theory. Pascal framed the choice as a wager—one that you must make, by virtue of being alive. It goes like this: God exists, or he doesn’t. Objectively speaking, none of us knows the probability of this binary. What we do know, and can control, is our response to it. Shall we believe and embrace religion? Or shall we assume that God is a lie and live without concern for the hereafter? It’s that last part that matters, according to Pascal. For if God exists, our choice for or against religion will have eternal consequences.

If God exists—and for now let’s assume we’re talking about the Christian God of Pascal—and you become religious, you stand to gain eternal happiness in heaven. If you forgo religion, your earthly life will be much the same as a religious person’s, though perhaps with more carousing. If God does not exist, the decision to believe or disbelieve has no serious implications for your earthly life and none at all for your eternal prospects, which are nonexistent either way.

If you are certain that God is real, the choice is clear: Be religious, not least because you have a soul to consider. But if you are uncertain, this is where probability comes in. Pascal argues that if you assign to God’s existence even the smallest likelihood, the choice about whether to be religious remains clear from a rational perspective. Only one of the four scenarios he described offers an infinite benefit (the eternal joys of heaven); the others offer merely finite pleasures. So, if you want to optimize your outcomes, it is best to choose religion. To borrow a term from decision theory, religious belief gives you the greatest expected value—that is, the probability of a positive outcome multiplied by its potential return. The smallest chance of infinite happiness conditioned on God’s existence beats finite rewards any day.

But if the math is clear, why do many of the most hard-nosed rationalists eschew spirituality? It turns out that there are a few good objections to Pascal’s analysis. For the most committed atheists, the probability that God exists isn’t small; it’s zero. There is no chance of eternal reward. The only happiness is earthly happiness. For other nonbelievers, the question is not so much whether God exists, but which God. Though most faiths have a notion of eternal reward (heaven, nirvana, jannah), there is always the possibility of choosing the wrong religion. Finally, there is this question: Even if God exists, and even if he permits people to worship him through many different faiths, is an instrumental approach to religion an acceptable one?

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