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[Reforged]

How religious are they? Because I'm guessing by the fact they just gave you a Bar Mitzvah that they're pretty fucking religious.
In which case you definitely shouldn't tell them till you're older and not dependent on them for anything.
You don't have to lie, just don't correct them.

I know its kind of a burden and I'm supposed to tell you "Clear your mind, tell them everything." but the fact of the matter is we've had some young people here who did just that and found themselves in a shitstorm that defies explanation.

If they're open minded about that kind of stuff then give it a whirl, otherwise just keep it quiet. Just because they don't know doesn't mean you can't discuss ideas with others or do research into stuff. When I first realized it was all bullshit it was because I actually took the time to read the Bible cover to cover. Then I wanted to learn more and more. Religion depends on the gaps in our knowledge and our unwillingness to accept them, if I were you I'd just focus on filling those gaps as much as possible by learning as much as you can about questions you feel haven't been answered. Keep in mind mankind as a whole only knows so much as of yet but that just means there's room for improvement.
The only thing we can ever truly know with absolute certainty is how little we know, the search for knowledge is a never ending but enthralling one.
Nothing is more important than knowledge and its something you can pursue without interference.
Freeing your mind is only the first step, you have to use it too.

Welcome to the forum by the way.

I want to tell them today but as i said before, i don't know what to say, i will end up saying; "iii uhhhh bye."

I'm very wary of advising either for or against that course of action.
On one hand it could go brilliantly. They could go "Well thats alright son, its your life and we love you no matter what path you choose." or they could go a very different direction.
Only you can know which is more likely.

How do they react to others not of their religion? Do they have people of different views in their social circle? How much importance do they place on their religion?
How do they respond to queries on the subject?
The answers to these questions may act as indicators of how they might react and will help you in making an informed decision.