If I told someone that lacking hair is still having hair, or lacking a hobby is itself a hobby, they'd probably ask whether I've been feeling OK and might even suggest counseling. They would surely doubt my ability to think and reason coherently. Such claims represent a complete separation from both reality and the most basic rules of logic: if something is absent, then it cannot also be present at the same time. By definition, something absent is not present — and that counts as much for beliefs as it does for hair and hobbies.
Some Christians try to get around this basic fact of logic by arguing that the absence of belief in gods entails the presence of the belief that gods do not exist — or put more simply, anyone who does not happen to belief the truth of some proposition automatically denies the truth of that proposition and asserts the truth of the contradictory proposition. Logically speaking, this position is even more unsound and invalid. It's completely devoid of any logical coherence or sense.
A belief is the mental attitude that some proposition is true. For every given proposition, every person necessarily either has or lacks the mental attitude that it is true. It is not necessary, however, to have the mental attitude that the proposition is false.
For example, if you do not quite understand a claim, then hopefully you won't believe it's true but at the same time you have no good grounds to believe it is false. In such a case, you lack the belief that the proposition is true but that's not the same as believing that the proposition is false.
