Anthropologists studying religion can you tell that every single human tribe or civilisation ever recorded believed in some sort of religion, with its god, or gods, and creation story. We all know about the ancient Greeks with their Zeus and Apollo, and the Romans with their Jupiter and Neptune, but don't worry these are all examples of ancient gods, right? Their time has been and gone.
I'm inclined to believe that if you told a person from long ago that happened to believe in say, the babylonian defeater of evil water gods and creator god, the great Marduk, that in a couple thousand years time some blogger will be citing him as an example of how socially constructed and fallible all gods are. And granted that the further back you go the more severe the punishment for blasphemy, then I would be certainly surprised if you lived to tell the tale.
We happen to be in a certain time and place right now, and the gods that are considered great, could possibly end up in some other well written blog post in centuries to come. Who's to say that Yahweh or Jesus, Allah, or Krishna won't be the same butt of a joke? And good luck to that witty person/AI.
Back to our anthropologists, if you were to kindly ask them to tally up the number of gods ever worshipped right up until today, then depending who you read or spoke to, there would be a great deal of variance from around 3000 to half a billion? Now even I find half a billion to be a ridiculous figure. Nevermind the point can be made with lets say, 3000 gods. We can confidently say that atheists do not believe in 100% of those gods. Now take your average monotheist and you can see that they are in fact 99.96% atheist, because they have to deny those other 2999 gods for their particular god to be well, the god. Atheists don't believe in all those old timer gods, and we also happen to take that 0.04% step further.