Believers often like to characterise atheism as a religion, I'm sure it makes them feel comfortable to tell themselves that we atheists just simply must have a religion too. There are plenty of serious and funny rebuttals to this point ('Atheism is a religion like not collecting stamps is a hobby' is one of my favourites), one of the more common rebuttals is to point out that atheism isn't really a belief system but a lack of a belief - a belief in gods. At the same time that believers throw this tired argument out they will often trot out the old 'You must have faith to believe there is no God.' Or 'Well you can't say there is no God because there is no evidence there isn't.' A variation I heard myself was 'You haven't been everywhere so you can't know there is no God.' Sorry, no prizes for being able to rebut that one.
Often the differences in various types of atheism have to be explained - implicit, explicit, weak or strong. The common responses from atheists shy away from the strongest assertion of atheism though - I know gods don't exist.
When someone asks me 'Why don't you believe in the existence of God?' I find the question no more intelligent, insightful or necessary than if they had asked me 'Why don't you believe in the existence of Danger Mouse?' I see no reason to believe that gods are anything more than fictional characters. Hell, there's more reason to believe in some fictional characters than there is to believe in gods. Some fictional characters are set in amongst real life times and places that were experienced or can be visited after all. Anyone ever been to Valhalla? Hades? Heaven?
There is no evidence for the existence of gods other than stories - be they written down in holy books or collections of myth, or expressed in the anecdotes of believers. And no, buildings built in their honour or deeds done in their name don't count, they just prove people will do anything for their religious beliefs. There is no evidence for gods outside of stories. Really my response when someone asks 'Why don't you believe in the existence of God?' Should be some variation of, 'Why don't you believe in the existence of Major Major Major Major?' With the alternating substitution of any random fictional character that happens to come to mind as the fancy takes me.
They're just stories people!
Even if the Bible contains the occasional historical truth that can be verified by an external source that does not prove that the supernatural content is true. If, for instance, the Bible is right about the existence of Pontius Pilate, that does not prove that someone called Jesus was the Son of God and rose from the dead anymore than the existence of the Empire State Building proves King Kong climbed it.
All of which eventually brings me to my point. I see no more reason to believe in the existence of gods than in the existence of the Nazgûl. I think that when we don't treat the existence of gods in the same way we treat the existence of other fictional characters we are inadvertently lending the holy books and myths some factual legitimacy they don't deserve, and this is almost certainly a by-product of us all being raised in religious cultures.
We are saying, with no good reason to, that those supernatural stories might be true. Why? We don't say the same about Star Wars and the existence of Darth Vader and the Dark Side. When we make up fantastic stories now they go into the fiction section of a bookstore - but if they are old and religions are based on them we (atheists in general) treat them like they might be true, we just don't know for sure so we await confirmation one way or the other. Why? Why do we have a term for people who don't believe in gods, but not one for people who don't believe in The Flumps? Why do I have to explain why I don't believe in gods when I don't have to explain why I don't believe in The Wombles?
I know that there are no gods. I know that there is no Gandalf. I think that there is no difference between the two.