For a long time nothing on Earth believed in God. This is because bacteria aren't capable of holding complex beliefs. But then humans came onto the scene, and the number of assumed Gods made a huge jump. We don't have cultural evidence for pre-civilized man, but the earliest religions we know about tend to be animist. Early man believed something supernatural resided in everything, from cats to thunder. Threads of this belief system persist today, but in general the number has been in decline ever since.
Take ancient Egypt and Greece as examples. In both cases the spirits in individual stones and animals were winnowed down into the major forces. Gods created the world and dealt with the dead. Gods were or navigated the sun. Gods designed animals. Gods put the kick into wine. Things that were overly commonplace, individual blades of grass, lost their mystical nature.
Polytheistic religions are more common today then animist ones, but the dominant belief system is monotheism. In one way, its a move full circle: an omnipresent God is in each blade of grass. But instead of everything having a unique spirit, everything shares one.
The Sun being pulled across the sky in a chariot seems a little silly now. We believe in gravity, and feel it explains the phenomenon much better. But despite a lack of sophistication in the vessel, the Greeks did understand what the sun was. They understood there was relative motion going on between it and us. They understood that it was very hot and that we really didn't want to be any closer or farther away. There were no words in Greek for plasma, nuclear fusion or gravity. Without those concepts, a God made the most sense. In a way, Helios on a chariot was a stand in, a black box to be filled in with better explanations later on.
Thus there's this pattern through history of observing a phenomenon, attributing it to the Gods, and eventually evicting the Gods in preference to science. Demonic possession made way for modern psychology. Bacchus has been reduced to ethanol. As the natural phenomena were demoted from mythology we were left with deeper questions: What created all this? What happens when we die? And these have been wrapped together into a single, almighty power.
I've been struck by all the parallels between the monotheistic (specifically Christian) God and the Universe. God is described as omnipresent, as the universe is by definition. God knows all, and if the Universe is deterministic it holds the keys to all that was and all that has been. We are created out of the cloth of the Universe, and return to it at the end of days (not that we ever really leave it). The scope of the Universe defines the beginning, and ending, of everything. And just as God is described as unknowable, as beyond the possibility of pure comprehension, so appears the Universe. If the speed limit of light holds, then we can only ever see and know a tiny bit of the Universe. It looks to expand far past what we could ever possibly experience, its own rules nullifying the possibility of comprehension.
Thus, I believe God is ultimately our struggles to comprehend the Universe we find ourselves in. And rather than God, I believe in that Universe. I don't think its a poor trade. I believe in a Universe far richer then the one we fathom. My faith tends to the idea that we're a tiny point on a ripple through an unimaginably huge, many-dimensional cloth. I believe that just as our world of ideas and society is uncomprehensible at the level of atoms, we're part of some other structure with its own elegant logic, but which is unlike anything we experience. Looking at the mundaneness of everyday life, I can see where people feel a life without God is lacking a crucical spark. But if we peer past the realm of the everyday, I really believe that the Universe provides all of the mystery, truth and beauty of any God figure.
An afterlife has always struck me as wish fulfilment. The use of God to explain the start of the Universe just pushes the question of original cause back another step. The bible reads much like other mythologies humanity has put aside. As you strip God of human desires and needs, he just seems to blend back into that Universe I believe he represents. This breaking of the dichotomy of Universe and God back into a single concept is where my faith lies.