"...the existence of an unseen force at work in human history is no less plausible and no less scholarly as an explanation for events than it is to consider the influence of unseen forces at work in the natural world (e.g. gravity)"...John S. Hatcher, The Ocean of His Words p 27
There is quite a world of difference between gravity and God.
Gravity, although it cannot be seen directly, can be measured through it's influences on other objects. We have built a gravity wave detector, and it is hoped it will detect gravitational waves before 2010.
Gravity is a physical force that is required to explain many different phenomena, such as why we stay on a spheroidal earth, the orbit of satellites, the motion of moving objects, and so forth. We cannot explain these phenomena in any other manner that answers all of the facts so easily.
Gravity does not depend on how you feel, or what you had for breakfast, or whether you believe in it or not.
On the other hand, God has no supporting physical evidence. It is not required to explain any physical aspect of the universe, except in addition to any laws of physics or chemisty that you need to explain anything. God's effects depend on very nebulous concepts such as God's will, and so forth.