The famous bargain by Blaise Pascal: compare what you stand to gain and what you stand to lose, by either believing or not believing in God. Consider the results if God either does or does not exist.
- Believe — If God exists, you gain eternal (i.e. infinite) benefits. You lose only some small extra cost of trying to live according to the way "believers" ought to live. If God does not exist, you gain little, only the somewhat better self-opinion of the believer and the thanks of those you help because you believe; the cost is the same as the above.
- Do not believe — If God exists, you gain nothing. You lose big time, reaping eternal (i.e. infinite) pains. If God does not exist, you gain the same nothing, but you also lose nothing.
A simple cost-benefit analysis indicates that believing is by far the better option if God exists, and it is probably a wash if no God exists. The weight is in favor of believing.
But do people do cost-benefit analyses about their beliefs? Nobody that I've heard of. You can reason it all you like, but believing that Pascal's bargain is in favor of God is not the same as believing in God. Pure factual believing gets one nowhere, as James wrote, "The demons also believe, and shudder," and we don't expect any demons in heaven.