Why do you believe that round squares do not exist? The popular idea of God contains self-contradictory traits, things which could not exist in the same being. For example, omnipotence and omniscience. How could a being be all-knowing and at the same time all-powerful? If God knows everything he's going to do in the future, he does not have the power to change his mind. If he has the power to change his mind, he does not know what he is going to do in the future.