Which Christian Church today stems back to the original Christian Church founded by Jesus Christ?
Almost all Christian churches today make this claim.
They also claim that all the other denominations have drifted away from the truth.
You could Google "true church history" to find articles from various churches that claim that the Roman Church (and its Protestant off-shoots) are not the original Christian Church, but that they are.
But you didn't ask "which Christian Churches claim that …?"; you asked which churches actually do, and that question does not have objective agreement.
The meaning of John 14:6 in relation to pre-existence of Jesus before the incarnation
"The meaning" implies that it is possible for everyone to objectively agree on what that meaning is.
That simply isn't possible.
I could tell you that some denominations believe that the pre-incarnate Jesus was the JHVH of the Old Testament, and that the patriarchs et al. dealt with him, and not with whom they now know as the Father.
But that wouldn't be answering the question as you asked it.
Why some believe that Jesus Christ is God and others don't?
"Why …?" is a very subjective question.
I cannot explain why you believe some of the things you do, any more than you can explain the reasons for my beliefs.
An objective atheist would answer such questions by saying that the reason is because we are delusional.
Is it okay to become a member or stay in the false Christian denomination?
"Is it okay …?" is a very subjective question.
Instead, say something like "How does the SDA church explain their belief that non-SDA Christians can be saved?"
Generally, you should ask questions as if the person you are asking is a religious scholar at the world's largest reference library on religion, but who is personally an atheist.
"Is the Pope Catholic?" is a suitable question, as it asks only for secular facts; "Is the Pope infallible?" isn't, as it asks for religious belief; but "Do Catholics believe in Papal Infalibility?" would be okay, as it asks about an objective fact.
Some other SE sites that deal with fiction (films, books, etc.) have the concept of "in universe" questions, which ask such things as why a character did something or how a seeming contradiction can be explained, all to be answered as if the fiction were taking place in a real universe. If answered "out of universe", the answers would be something like "to further the plot" or "because the author made a mistake".
Most questions for this site implicitly make that same distinction, but the example questions above are all asking for out-of-universe answers to in-universe questions. That's why they were rejected; not because they were intrinsically bad, but because of the way they were asked.