There were no beliefs until theism developed (in all the various forms, based on whatever was made up over tens of thousands of years by the various factions). The theists tried to spread their message. Those who rejected theist doctrine became known as "atheist" - having the Greek "a" added to "theist" to make a new word to mark the fact of being "not at all theist in any way, shape or form".

Unfortunately, the popular view among theists is that atheists are as Joe describes i.e. agressive in their non-belief, desperate to prove that the theists are wrong etc etc blah blah. The fact of the matter is that true atheists simply have no interest in theist issues. "Aggressive atheists" - I would guess a tiny minority - have some other agenda altogether, and may well be known as "anti-theist", rather than "a-theist".

I think the reason atheists (or antitheists) get a bit short with theists is because it seems that all the world's problems seem to be based on religious differences. My view is if I'm going to be made to suffer in some way, I don't want it to be because the imaginary friend of the society I live in is different from - therefore deemed invalid to - the imaginary friend of some other people in a different society.