If you dabble in Christian Philosophy you will come across Pascal's Wager. You have likely thought it without knowing it was formally named. Its power is probably in its simplicity.
Blaise Pascal died in 1662 at the age of 39. Aside: Niklaus Wirth's 1970 PASCAL computer language was named after Blaise Pascal since he invented the adding machine three hundred years earlier.
His French, Catholic theology reflected the 17th century Jansenists who themselves reflected John Calvin, a Protestant reformer.
Pascal, in his posthumous writing Pensées, rejected today's wildly popular Natural Theology (theologia naturalis) which seeks a proof of God through reason and evidence, not revelation.
He believed "there is a God, and there is a corruption of nature which makes men unworthy of him." Reason, to Pascal, only demonstrated man's own finiteness.
Nonetheless, you'll see here in a second that his Wager is a matter of pure reason. Nobody is pure anything, are they?
So, Jerry, what's the Wager?
What Pascal demonstrates is not that God exists, but that man ought to believe in God's existence. Though this may sound in conflict, the reasoning is simple and clear.
Pascal's Wager opposes the finite loss inherent in believing in God's existence versus the infinite loss supposed in the denial of God. In other words, the Christian may be right, but if he is wrong he only loses the opportunity for a wild life. The atheist may be right, but if he is wrong he forfeits eternity. That is the wager.
The obvious problem that even Pascal recognized was that the wager did not necessarily lead to the Christian God. Nonetheless, that's Pascal's Wager. It's a decision most believers have made subconsciously without knowing Pascal ever existed.