Given any proposition, anyone can say that the truth value of the proposition is "true" or "false", even if the "proposition" is "all gronks are grankled" (the scare quotes surrounding "proposition" are because, in the philosophy of logic, whether statements are actually propositions if they are identical to propositions syntactically but are meaningless is a matter of debate).
More importantly, even were I to provide a formal proof, I can't use classical logic as it is impossible to do so for the evaluations of propositions that include mental state predicates, and there is no formal logic that relies upon the naïve, simplistic binary-valued logic you implicitly do for mental state predicates like "believe".
Put simply, your position contradicts classical and non-classical logics, and is logically untenable.
However, as I can't rely on logics you don't know to prove this, I use examples. It seems intuitive that if you don't know if X is true that you neither believe X or disbelieve it, but it also seems intuitive that if you don't believe X, then clearly you "must" believe that X isn't true. This is the fallacy of equating propositions in sentential logic with modal logics or many-valued logics (while the latter lack the canonical status that classical predicate/propositional logic have, classical logic disallow statements of "belief" as possessing truth value).
What if I believe daimon exist? What if I believe that Jesus existed, and the proposition "Jesus was the son of God" is true?