I'm not sure believing in God is as simple as 'if you do, Hooray! You get into Heaven!'. There are more consequences to look at. People fear what they don't understand, and one omnipotent being who has existed forever and created everything is definitely beyond comprehension. Supposedly, a good person is 'god-fearing'. As far as I can tell, people don't like living in fear. Fear of judgement is another cause of anxiety. Having a God who sees all, knows all, all the time is more than a little scary. I doubt people would live life to its fullest if they were aware that at all times they would eventually be held responsible for their actions. It's not really a good feeling to be less than a bug, having not only your life of say, 60 years, and also the eternal existance of your soul to be in the hands of a being so far above you, you may as well be an insignificant insect. Looking into religions, fear itself may be the cause of why they start wars. It could be that religious people, fearing that they did not choose the 'correct' religion, or that they did not do what would please their God decide to destroy all those whose beliefs differ from their own.
Religion is a way to gain power. Look at the United States' government. Peoples' personal religious beliefs affect the world in so many ways. (Abortion and stem cell research for example.) During the Crusades, would it really be possible for Jerusalem to be conquered without the Pope telling the armies that it was the will of God to do so?
And then, with an omniscient being, who can say if they'll accept that you believe in them only because the alternative is the risk of eternal damnation?