Its really not too complicated. For example either God exists or he doesn't. Either Jesus rose from the dead and is the son of God or he is not. The criteria is that it corresponds with reality and what actually happened.
The only religion I know that is one hundred percent true is the one I follow. It's spiritually true. It's logically true. It's true by my experience as well as others. It's just, well, true.
That is, since you said one religion, not the "christian" religion.
To put it blunt, any religion that says the supernatural is real whether it's a dragon in the sky or a man floating above the moon is making a
claim not a fact. I don't believe in claims. "Jesus existed and he will save you from your sins" is just a claim or statement. It means nothing to me. It's not a fact. There is no certainty. It's false.
Unless someone can point out that Jesus saves people without anyone needing to explain it in religious jargon, I don't see it as a part of reality
outside of the believer's experience.
So, does Jesus exist? Yes, in the believers experiences.
Does god exist? Yes, in the believers experiences.
Ask any believer to proof god and Jesus (as god) exist outside of their experiences, they may give you synchronicity and things they put together with the assumption of god being the center of it all.
There is nothing wrong with that. They have absolute 100 percent certainty based on their own criteria, experiences, and claims.
In general, no. I don't agree Jesus exist as god and god exists. There is too much evidence, if you like, to prove they don't that the only thing that holds true to the believers that it they are certain is faith, obedience, tradition, and experience (not limited to).
If you're looking for an answer to an equation, there aren't any. How can you find certainty or the opposite of something that doesn't exist to even question it beyond the believer's claims?
Why would we do so unless we are curious, want to study it, or maybe convert, or find revelation in it? I don't know.
I just
know I am certain about my practice. I only
believe that people are certain about theirs. I can't know at all not even one percent because there is nothing outside of the believer's experiences and claims that would make me think otherwise.