Say that one day a strange person comes to me and say: "There's 50% chance that there's a suitcase full of money in the other room of this house. If you believe in the fact that the Earth in flat, then I will give you the right to open the door and take that money for yourself if it happens to be there".
My thinking will be as follows. First of all, the fact that there's a suitcase or not is clearly independant from my own will or whatever I say. So by myself I cannot do or say anything that will make that suitcase appear (or disappear). With this in mind, if I say that I do not believe that the Earth is flat, then I will just not be given any chance to open the door, but if I say that I believe that the Earth is flat, then I will be given this chance and if I am lucky I will get the money. Therefore, it is better, at least as far as we accept that having money is better than not having any, to say that I believe that the Earth is flat.
An interesting question now is: Has the fact that I said that I believe that the Earth is flat made the Earth flat ? The answer is that it clearly has not. I may claim as loud as I can my belief in the flatness of the Earth, it is not and will not become flat just because I said so.
An even more interesting question is: Does the fact that I said that I believe that the Earth is flat means that I actually believe that the Earth is flat ? The answer is no, it doesn't. I may have said that I believe that the Earth is flat, but only because I was interested in the advantages (a chance to open the door) brought to me by making such a claim.
Another important thing is that assuming that I didn't know whether the Earth is flat or not, basing my belief on a potential gain, would not have been a true belief. It would just be a materialistic and pragmatic belief.
A true belief, is a belief that one's mind would genuinely generate out of some true insight on the nature of reality. A conviction that one would not be afraid to check against reality (if such experimentation is possible).
Another way to put it is that true beliefs are motivated by nothing else than a search for the truth (whatever it might be), and the acceptance of the truth if it becomes available. In other words, belief, is a temporary state in the search for the actual truth.
So now, should I really take time to say how much I am pissed off by religious people who base their belief in God on the fact that "If God happens to exists, it would have been better to have previously believed in She". If God exists, I am sure that She will prefer people like me who are more interested in the Truth in a genuine way (even if I may have missed it), than by people interested only in "potential gains".