"Are the Jehovah's Witnesses' your definition of "believer"? I believe they read the same Bible you do. Who, in your opinion, decides what makes a person a believer?"
What does a JW have to do with my statement?
Back on topic - the "church" is the body of believers. That is how the Bible defines the "church" and the Bible IS the Word of God. I believe God defines who believers are not mere fallible mortals of any stripe.
You are in error with your belief unless you are talking to another Jehovah Witness, since their bible is a man made variant of the Christian text.
Assuming you are talking to a 'protestant' the new testament is shared with Catholics and the protestant old testament is shared with the Jews.