Pascal's Wager (or Pascal's Gambit) the name for an idea that Blaise Pascal had. He said that it is not possible to prove or disprove that God exists. Therefore, it is better to bet that God exists. If God existed, and the person believed in god, he would be rewarded (with happiness forever); if the person did not believe, he would be punished (with what is called eternal damnation). If God did not exist it would make no difference. For this reason, it would be better to believe in God, Pascal said. Indeed, Pascal strongly believed in this ideal. Pierre de Fermat along with himself created probability theory.
This idea was very important for the time, because it did new things with probability theory. It is also one of the first tries to use the concept of infinity, and the first use of decision theory. It was important for other philosophers who developed the ideas of pragmatism and voluntarism.