This has been discussed several times. Quick C&P for you comrade:
The argument iknown as Pascal's Wager. It has several flaws.
Firstly, it does not indicate which religion to follow. Indeed, there are many mutually exclusive and contradictory religions out there. This is often described as the "avoiding the wrong hell" problem. If a person is a follower of one religion, he may end up in another religion's version of hell.
Even if we assume that there's a God, that doesn't imply that there's one unique God. Which should we believe in? If we believe in all of them, how will we decide which commandments to follow?
Secondly, the statement that "If you believe in God and turn out to be incorrect, you have lost nothing" is not true. Suppose you're believing in the wrong God--the true God might punish you for your foolishness. Consider also the deaths that have resulted from people rejecting medicine in favor of prayer.
Another flaw in the argument is that it is based on the assumption that the two possibilities are equally likely--or at least, that they are of comparable likelihood. If, in fact, the possibility of there being a God is close to zero, the argument becomes much less persuasive. So sadly the argument is only likely to convince those who believe already.
Also, many feel that for intellectually honest people, belief is based on evidence, with some amount of intuition. It is not a matter of will or cost-benefit analysis.
Formally speaking, the argument consists of four statements:
1. One does not know whether God exists.
2. Not believing in God is bad for one's eternal soul if God does exist.
3. Believing in God is of no consequence if God does not exist.
4. Therefore it is in one's interest to believe in God.
There are two approaches to the argument. The first is to view Statement 1 as an assumption, and Statement 2 as a consequence of it. The problem is that there's really no way to arrive at Statement 2 from Statement 1 via simple logical inference. The statements just don't follow on from each other.
The alternative approach is to claim that Statements 1 and 2 are both assumptions. The problem with this is that Statement 2 is then basically an assumption which states the Christian position, and only a Christian will agree with that assumption. The argument thus collapses to "If you are a Christian, it is in your interests to believe in God"--a rather vacuous tautology, and not the way Pascal intended the argument to be viewed.
Also, if we don't even know that God exists, why should we take Statement 2 over some similar assumption? Isn't it just as likely that God would be angry at people who chose to believe for personal gain? If God is omniscient, he will certainly know who really believes and who believes as a wager. He will spurn the latter... assuming he actually cares at all whether people truly believe in him.
Some have suggested that the person who chooses to believe based on Pascal's Wager, can then somehow make the transition to truly believing. Unfortunately, most atheists don't find it possible to make that leap.
In addition, this hypothetical God may require more than simple belief; almost all Christians believe that the Christian God requires an element of trust and obedience from his followers. That destroys the assertion that if you believe but are wrong, you lose nothing.
Finally, if this God is a fair and just God, surely he will judge people on their actions in life, not on whether they happen to believe in him. A God who sends good and kind people to hell is not one most atheists would be prepared to consider worshipping.
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