As an atheist I believe there are no supernatural entities. Just rearrange the sentence.
Rearranging the sentence changes it's meaning. Let's try and illustrate this a different way.
Let's say you are going about your business one day and then someone comes up to you, points at some guy across the street, and asks you what your state of belief is regarding the proposition that that guy has exactly 37 cents in his pocket.
What's your answer?
Do you believe he
doesn't? Based on what are you holding the belief that he does NOT have 37 cents in his pocket? Have you looked? Is it unreasonable to think there could be 37 cents in some guys pocket? You have no idea who this guy asking the question is, do you know he has no way of knowing how much change is in that guy's pocket so that 37 cents value in his question wasn;t just some random guess?
Or do you simply lack any particular belief that he does have 37 cents in his pocket because you've been given no reason to adopt that belief?
Clear that there's a difference now? "Not believing" and "believing there is not" are two significantly different things. Atheism is not believing a deity exists. You can ALSO believe a deity does not exist, but it's the "not believing" that defines atheism.