Blaise Pascal did not believe that reason alone could justify belief in a god. He came up with the following wager as an attempt to decide whether or not we should believe based on the possible outcomes for the afterlife.

Say that God exists. This god created the universe and everything in it including us. When we die, God selects some of us for eternal paradise and others for eternal torture. Of all the creatures on Earth, humans are the only ones that God gave intelligence, so obviously God wants us to use our intellect and to be freethinkers. So when we die, God will reward freethinkers with eternal paradise and punish the people who through away their reason by sending them to hell.

The wager is this: If you abandon reason and believe in God and it turns out that she exists, you are sent to hell in the next life. However, if you apply your intellect and withhold belief, and God actually exists, you get to go to heaven in the next life. Furthermore, if God does not exist and you spent your life believing in her, you have sacrificed your integrity and your self-respect and you have spent a lifetime helping to hold back human progress. So if God exists and you believe in her, you lose. And if God does not exist and you believe in her, you lose. Therefore we should all disbelieve.

Christians have more to worry about here than theists who do not believe in the Bible. By claiming that the Bible is the word of God, Christians accuse God of ordering mass murder and approving of rape and slavery (see Bible Atrocities). This may deeply offend God and she may punish Christians more harshly than others.

Wait a minute. Pascal didn't conclude that we should all disbelieve; somehow he concluded that people were better off if they did believe. Oh yes, now I remember. Pascal postulated the existence of a perverse god that sent freethinkers to hell and rewarded irrational believers with eternal paradise. That's why he got the conclusion backwards.

The problem with the above arguments are that they use a false dichotomy. That is they incorrectly restrict the large number of possible situations to just two. A particular god exists or it doesn't. This ignores the fact that we could have just as easily postulated the existence of any number of gods each with her own criteria for salvation and damnation. Say that the possibilities in Pascal's wager are that the "Freethought God" described above exists or that the god of Christian mythology exists. What is the conclusion of Pascal's wager now?

For more on Pascal's wager, see the Secular Web's Refutations of Pascal's Wager.