Beliefs are propositions that can be either true or false, but they are not simple things that you either have or do not have. For one thing, they are scalar in nature. That is, there are degrees of certainty. If you are unable to decide whether a proposition is true or false, that is a very different mental state than having no proposition to decide about at all. Indecisiveness is not so much lack of belief as it is inability to suppress one of the two truth values that we assign to a proposition. Sometimes we call that "being of two minds".
I think of the mind more as having a mass of competing beliefs rather than just a set of propositions with fixed truth values. Every proposition--a statement that could be true or false about reality--is up for grabs. The belief that gods exist could be very weakly held on either side. A person can lean positive or negative. If that person leans negative, no matter how weakly, then I consider that person an "atheist". If a person can't decide from one moment to the next, then I wouldn't call that person an "atheist". If the person has no concept of a god, I would consider the question of a label moot.