Many people have not believed in a god at all times in history, and whether they have or not is of little significance. Other things are of much greater importance. "God" is after all a word and so of far more significance is what this word signifies to them when we say that they do or do not believe that what it represents exists. Thus in some ways this comparison of "those that do" and "those that don't" is a somewhat meaningless comparison.
This juxtapostion with science and the theory of evolution is also very "interesting". Those who actually work in real science rather than merely paying lip service to it (whether to claim it proves their god or gods exist or do not exist) have a dedication to discovering the truth about things. That is something I certainly think is more important than this "God" pretend-issue. I certainly know that people other than scientists can also have a dedication to discovering the truth about things, however those that put themselves in opposition to science cause me to seriously doubt whether this is true in their case. For in their case, I suspect that what they are doing is fighting for their "truth" -- willfully trying to impose what they have accepted or decided is the truth on the world rather than honestly or humbly seeking to discover the truth. But the same is quite often true of those that uphold science as a source of truth, for paying lipservice to science, they often see it as a means to push what they have accepted or decided is the truth on others. Thus a far more significant distinction than that between "those that do believe in God" and "those that don't believe in God" is between the seekers after truth and the pushers of truth.
I suspect that one of the main differences between many of "those that do" and "those that don't" is whether the truth and the search for truth has any part of the meaning they give to this word "God". Many differ in they type of experiences they have had with "believers in God", such as whether the believers in God that they have encountered were "seekers" or "pushers". Another big part of this question, however, is methodology. How do we discover the truth? Experiment and observation? Reading old texts? Prayer? Staring at ones belly button and proper breathing? Babbling over drinks in a bar? Debating with others over the internet? Using your imagination? Working in a creative endeavor like art, writing a book or making a movie?
Well the effectiveness of a methodology depends very much on what one is seeking the truth about. For all that people of today have become enamoured of science because of its effectiveness in discovering the truth, the fact is, and I tell you this as scientist myself, the method of modern science is based on premises and assumptions which the scientists themselves can clearly see limit the kind of things that it can discover the truth about. One of the most fundamental priciples of modern science is an objectivity that is obtained by making itself oberserver independent by requiring that its data be obtainable by anyone following a certain proceedure. This is one of its most important techniques for getting past the beliefs and assumptions of individuals to the actual truth about things. However this method has a fundamental flaw in that it makes itself blind to any aspect of reality where the observer necessarily has an impact on what he observes -- such is most clearly the case when the subject of discussion is the observer himself and the nature of his own existence.
The point here is that we use different methodologies because we are interested in different things, and it is my conclusion, from my observation of the discussion between people that a vast portion of the differences between peoples beliefs about things is a difference in the sort of things they are interested in. I have seen so many arguments/debates on the internet where when you look at it carefully they are mostly just ignoring each other in order to continue talking about what they are most interested in.
Now for myself. I am a Christian as well as a scientist, but not only was I not raised a Christian but I was a scientist before I was a Christian. This made me rather immune to all the anti-scientific trends within religion, because it could never have been a choice for me between science and religion. The question was always one of whether there was any value for religion or a belief in God when the value and truth of science is a given. Modern science has become a fundamental part of my perceptual process by which I see the universe which I inhabit for what it is and so I could no sooner disregard or reject it than I could choose not to see or hear.
But perhaps you can see why the important question for me was never "whether God exists" but "what is God", because it made no sense to ask whether something exists when you do not know what it is. Or more importantly, the idea that people spent so much time talking about something that did not exist seemed a bit absurd to me. The question was thus always one of understanding what they were talking about. Is God a myth, a person, a fairy tale, the creator, a story book character, a ruler, a delusion, a parent, or what? I had no answer to this handed to me on a platter, certainly no answer that was very coherent, and definitely not an answer which I found acceptable.
I remember comparing the ideas of many different religions on the topic. But what finally gave the word meaning to me was a decision that a "faith in God" was somehow equivalent to a faith that life was worth living. What this means is, that you cannot have a faith in God without a faith that life is worth living and more importantly that if you have a faith that life is worth living, then in some sense, no matter what words you might use for it, or what you might call the object of your faith, you essentially have a faith in God.
You see part of the problem is that the word "God" has a history of use and abuse and thus this gives rise to situations where an individual may be forced to repudiate "God" with all the life-denying baggage it has been loaded down with in their life, in order that they can make a real and effective affirmation of life - but a true affirmation of life is an affirmation of the true God, for as I have now come to believe, life is God's creation and you might say His "obsession". It is for life that God created the physical universe and it is for life that God has always worked and acted, encouraging living things to reach out for the potentiality that is within them and for life in general to reach out for the infinite potential that it is ultimately capable of.
And so what are my conclusions now about what God is? Is God a myth? yes. Is God a person? yes. Is God a fairy tale? yes. Is God the creator? yes. Is God a story book character? yes. Is God a ruler? yes. Is God a delusion? yes. Is God a parent? A big yes. Yes for some people God is a delusion and a part of their psychopathology. Yes God is a fairy tale to the simple minded of both the believers and the non-believers. But for me God is the creator and parent of life, for life is by its very nature is not something that a moral (or rational) being would create as a tool (because living things do not make good tools), but would only create life with the interests of a parent as an end in itself. Thus I have come to see God as the infinite being whose perfection and lack of limitation provides Him with only one rational motivation and that is to give of His abundance to others in perfect self-less love.