The significant difference here is that we can observe certain things that we don't understand (gravity for example) and have no choice to accept it. We can see and feel it's presence.
You cannot say the same about a God. One has never been felt or observed. Even if you think you feel or see God, that's just an interpretation of an experience. When every single person that has ever lived has seen and felt the effects of gravity, it's hard to make a case against it.