This verse gets into the complexities of the "if/then" conditions in Greek. The form doesn't say that the person won't believe, but that they probably won't.

The "if" is not the simple "if" in Greek but the Greek word meaning "if might" indicates more of an expectation of something happening than "if" alone. This is often how we use the word "when".  This word is used in two forms of "if/then" statements, one of certainty when used with one verb form in the then clause and one of probability when used with another verb form.

"I tell" is from the Greek verb that means "to say" and "to speak" also. The form is one of possibility, which is always used with the "if" conjunction.

The Greek pronoun "you" here is plural and in the form of an indirect object, "to you", "for you", etc. 

The Greek word translated as "ye will...believe" does not apply to religious belief as much as it does trusting in other people, especially their word. Christ usually uses it in contexts, as the one here, that apply to trusting words. The verb is in the future tense. This form of if/then indicates something that usually  happens.

The "not" here is both of the Greek negatives used together. Greek has two negatives, one objective, one subjective. The use of both together is more extreme, like saying "you cannot really think" or "never". The use of this double negative is especially entertaining because the verb can take either the objective or subjective negative depending on the meaning. You would think it would only take the subjective because we think of "belief" representing  thought, but the idea of "trust" is more than a belief. It can be the basis of actions. So the negation of "belief" with the objective, instead of subjective negative, equates trust with a fact. While the negative with the subjective equate trust with a feeling. The use of both? Both fact and feeling.