Pascal's Gambit or Pascal's Wager, is a rather famous rhetorical ploy. It is not an argument for 'god', instead, it attempts to outline the pragmatic benefits of believing in a god. As such, it is intriguing because it is intended to gain believers without having to solve the impossible problem of offering proof of God's existence. Therefore, Pascal's wager is alluring in its simplicity. It's really an attack on doubt, for it states that the requirement of certainty in belief is a false one. Either 'God' exists or 'he' does not. If 'he' exists and you are a beliver, you "win". If 'he' does not exist and you believe, nothing is lost. However, If 'he' does exist and you are not a believer, you lose out on eternal life. Of the four possible permutations:

Belief and God exists = believer is saved

Non Belief and God exists = non believer suffers hellfire

Belief and God does not exist = believer does not suffer more than non believer

Non Belief and God does not exist = non believer does as well as believer

...there is no place where a non believer benefits over a believer and in no case does belief ever bring harm. Therefore, logically, one should be a believer in god, if for no other reason than that atheism cannot ever benefit a person regarding the afterlife.

Seems pretty solid at first blush. However, this argument is specious, for many reasons. I will list several:

1) Pascal's Wager is rhetoric. The wager makes an appeal to adverse consequences and is intended to circumvent reason. You're supposed to experience fear (or gluttony for reward) and react emotionally. As such the wager is not an intellectual grounds for belief so much as it is an attempt to extort belief through fear and/ or by way of greed. In short, it relies on inculcating moral or intellectual cowardice in doubters. So, even if the wager were granted at face value, it would only establish the desirability of belief, "not provide grounds for belief itself." Pascal himself recognized this limitation and argued that one should use ritual and church attendance as a way to "deaden your acuteness," and eventually attain belief. In other words, he was advocating irrationality and conceding that his wager was an rhetorical ploy and not a logical argument. The fact that the creator of the wager concedes this is lost on those who continue to advocate his wager.

2) Point 2 works hand-in-hand with point one: Belief is not volitional. One cannot believe what one does not believe. Again, even Pascal recognized this and advocated ways to alter beliefs through 'deadening (intellectual) acuteness". Therefore, all Pascal's wager could generate is sham belief driven by prudence. One must be driven to ask: Would a moral 'god'really 'want' a hollow religion? Would he want followers who only hold to sham beliefs, driven by fear or greed? I would maintain that even religious people would reject Pascal's wager for this reason. Therefore, it is likely that atheists, 'agnostics' and theists can all agree that Pascal's wager is a vast oversimplification of a very difficult decision.

While the wager is already defeated at this point, every theist argument I've ever seen commits multiple errors, so we must continue:

3) Pascal's wager does not tell us which god to follow. It only presumes one should be a christian because Pascal was a christian. One could be a believer in a christian god and still suffer hellfire from Allah or Zeus.

Therefore Pascal's wager commits the false dichotomy error and is invalid.

4) Pascal's argument does not offer proof that disbelief in God means one is banished to hell. It is only, again, an assumption in some religions, such as christianity.

5) Pascal is wrong to assume that atheist does not benefit from his decision, if god does not exist.

A person accepting a false religions DOES have a lot to lose - you lose your own ability to make choices, as well as fail to appreciate the world in a different, perhaps relativistic manner. (See secular humanism for more.) If God does not exist, and there is no afterlife, we should be devoting our energies towards extending lifespan, and this requires a scientific, not a theistic, worldview. We also have reams of historical documents showing that theocratic systems have brought about the greatest miseries in history. Clealry, one has a lot to lose by being a believer, and accepting false tenets from false religions.

6) Pascal may be wrong to assume that the atheist does not benefit from his decision even if there is a god!

Monsignor Lorenzo Albacete states that the Roman Catholic church's official position on salvation is that what matters is that a person be true to his beliefs. He declares that an atheist can 'go to heaven' granted that the atheist is in earnest about his atheism. Read this page for more:

http://meaningoflife.tv/video.php?speaker=albacete&topic=complete

From the page:

According to the New Testament, Jesus was born as a sign of God's love for humanity--sent to Earth so that "whoever believes in him may not perish but may have eternal life," as the gospel of John puts it. Over the years, this prerequisite for admission to heaven--believing that Christ died for your sins--has been a strong incentive to become or remain a Christian. But if God really loves humankind, shouldn't He let, say, a good Buddhist or Jew through the pearly gates? God goes further than that, says Monsignor Lorenzo Albacete in this clip from his meaningoflife.tv interview: even atheists are eligible for salvation. This radical reinterpretation of scripture, Albacete notes later in the interview, has now become official Catholic doctrine (unbeknownst even to many Catholics).

7) In a similar vein, there is also the atheist's wager: http://www.infidels.org/library/modern/richard_carrier/heaven.htmlSeeing as no moral god would want people to accept him out of mere prudence, it makes sense to presume that those who are honest in their beliefs, even in the face of threats, will be rewarded

WHAT HAPPENS TO YOU IF YOU DON'T BELIEVE IN GOD? Which God?  How many gods?

Those who know the good, do the good. - Socrates