If you don't believe in God and God doesn't exist then you have a lifetime of being right. However, if you do believe if God and God does exist, you get a lifetime of being right plus eternity of bliss, which is much, much better.
If you do believe in God and God doesn't exist then you have a lifetime of being wrong. However, if you don't believe in God and God does exist, you get a lifetime of being wrong plus an eternity of damnation, which is much, much worse.
Therefore, in the absence of evidence one way or the other, you should probably gamble that God does exist as the pay-offs are so much better.
Of course, this assumes that people who believe in a god choose the right one to believe in. If they choose the wrong one, it's not so great:
