When one gets ready to believe that there is nothing like God, he gets haunted by the thoughts like "after death" and "before birth". Then a person feels as if life is merely a biological event. And his existence is nothing but an infinitely small fraction of a moment in eterntiy. This drags him back to believe that life is human centred and there exists a God whose form resembles to us.
When the religion talks about life after death, incarnation, reincarnation, moksha,nirvana 'n all - i doubt it.
Here again the above person is forced to believe in life after death and the greater philosophy of life [which says that life on earth is a shadow of the eternal radiation].
Unless a person is brave enough to believe that life is nothing but a biological event and his existence is not more than his life-span, he would surely believe in the above philosophy.
One other fantastic theory that strikes my mind is that some of our ancestors were great souls and they knew exactly what kind of animal a man is. He is always unsatisfied and wants more than he has. He is intelligent whose intelligence increases by each passing moment. And he will lose morality unless ruled by an eternal king. So these sages created an imaginative character of God and made him responsible for all the things and occurences around us. They were excellent orators and so gathered a big crowd around them and said " Look, if you want to harm others do it. But remember God is making a note of all of your actions and you'll suffer in your life further. And also you will go to hell or heaven on the basis of your actions." These great souls then created a code of conduct that every man should follow to reach heaven. You will see the base of implementation of each religion is fear, the fear that you'll go to hell, you will be punished by God, you will not attain moksha, you will suffer in next birth, you will ...... if you harm others. If we represent them in this way : "Do good, you'll go to heaven" then it is simply changing the grammatical form of the statement but the root remains in the same soil of fear as it was in the above statements.