Pascal's Wager is the argument that you should believe in God, even if there's a strong chance that he might not be real. The claim is that you should believe in God just because there's a chance that you might go to heaven and avoid hell. Based on simple probability theory, the argument was first formally put forth by Blaise Pascal, a 17th-century philosopher and mathematician.
God might or might not exist. It is a gamble whether you believe in him or not. As with any gamble, we should consider the odds.
Pascal described the payoff of this gamble as follows: If God does not exist, then you neither gain nor lose anything from belief or disbelief. In either case, you just die and that's the end. However, if you choose to believe in God, and you are right, then the reward is infinite — eternal bliss in heaven. On the other hand, if you choose not to believe in God, and you're right, you gain nothing. But if you are wrong, your payoff is negative infinity — eternal suffering in hell.
|Payoffs||Believe in God||Don't believe in God|
|God doesn't exist||0||0|
|God exists||+∞ (heaven)||−∞ (hell)|
Since the chance of God existing is unknown, but the payoff/punishment scheme is infinitely in favor of believing in God, just on the small chance that he might exist, you'd better believe. It's the only wager that makes sense.
Nonzero payoff for atheism
In the case where God does not exist, there really is a clear advantage to not believing. In other words, the payoff is not zero.
For one thing, if you go through life believing a lie, that is a bad thing in itself. Besides that, there is more to being a believer than just saying "Okay, I believe now" and getting on with your life. Serious believers spend a lot of their time in church, and contribute a lot of money as well. There's a reason why some towns have very affluent looking buildings for churches, and why large and elaborate cathedrals are possible: they're funded by folks who donate a tenth of their income throughout their lives to tithing. This is surely quite a waste if the object of worship isn't real.
That's to say nothing of the persecution of other groups that's been instigated in the name of God throughout the ages. Also, in the US, churches don't have to pay taxes, which includes property tax. Property tax is what goes to schools, so all the land that churches own is sucking money out of schools.
Even if you buy into Pascal's Wager and decide you should believe, that doesn't give any basis for choosing which religion to believe in. Fundamentalists often use the wager to prove that you should be a Fundamentalist, but of course, Pascal was Catholic and was using it to prove you should be a Catholic as well.
This just highlights the whole problem of which religion is the right one. Since many Fundamentalists believe that Catholics are going to go to hell, Pascal's not much better off than an unbeliever. We don't know if the Jews are correct, or perhaps the Muslims, or if reincarnation is right.
The faith of the wager
Even if one assumes that the wager applies to the Christian god, would he really accept the kind of faith it promotes? The wager doesn't promote true, deep faith; it promotes a fake faith. The person simply believes because they're afraid of the punishment for not believing. The wager is simply an attempt to force the person to believe (see argumentum ad baculum).
Where does it end?
If you can accept Pascal's Wager as a realistic reason to believe, that leads you to a point where you have no choice but to believe just about everything on the same grounds. Why even assume that God supports any existing religion? Perhaps there is a perverse God who likes critical thinkers and doesn't like "blind" followers. Perhaps all atheists will eventually find themselves in heaven — completely surprised, but pleased anyway — while their Christian counterparts burn for eternity!
Lacking specific evidence about the nature of the true religious faith, there are an infinite number of possible requirements for going to heaven and avoiding hell. Maybe only those who collect stamps go to heaven. Maybe you have to donate $10 a week to Iron Chariots for life. Why quibble about a few measly dollars if it will save you from eternal hellfire?