If your answer to the question 'do you believe that at least one deity exists' is anything other than 'yes', then you're not-a-theist, and therefor an atheist.
Even, or maybe especially, if that answer is 'I have no idea whether or not deities exist'.
People tend to either over-simplify or over-complicate things needlessly. It's a binary set - you're either a theist or an atheist. Even people who believe in spirits and reincarnation and astral traveling and all that sort of stuff are 'atheists' if their beliefs do not include a deity per se. There are lots of kinds of theists - monotheists, polytheists - even pantheists. There are theists who believe that there's at least one goddess, rather than, or in addition to, at least one god. Similarly there are many kinds of atheists, from 'spiritual but not religious' to crystal-pyramid-chakra-aligners to Ouija-board mediums to faith-based atheists and everything in between. Some theists don't like being lumped in with Muslims. Some atheists don't like being lumped in with certain Scientologists and Buddhists - each of whom has varying views on 'God' depending on who you ask.
Both are really 'big tents' with wide ranges of viewpoints and beliefs on either 'side'.