You seem to misconstruing ideology with reality. The idea of a god may affect others, just as an engaging novel or television show would, but that does not mean the characters in your favorite novel or tv program exist. A stone of measurable mass and density may be proven to exist by showing it's physical effects on it's environment. Which includes but is not limited to properties such as gravity, hardness, and reflected light. However the rock is not an idea in one's head. A god on the other hand, is a vision of man and does not exhibit direct physical properties.