If there were no Holy Book in existence--what would you or anybody else KNOW about God and how would you know it?
Those of faith would know about God according to what they hear from Him and from the Spirit (Christ), same as they did to begin with before there WAS a 'holy book'. Those people would share what they hear, and others might also come to listen and so hear, and believe. Still others would corrupt the message and truth, and mislead many.
Same as what happened from the beginning, in truth.
If whatever Holy Book(s) turned out to be unreliable for any reason (or at least hit-and-miss) would that be any different from being totally wrong?
(Example: You have a list of important things that will hurt you, help you and waste your time--but--they aren't categorized which is which!)
That is what we DO have... though being errant does not make something useless or totally wrong.
You can learn what is what by experience... and testing. Just means that you think and reason and test what a witness WROTE (or a witness to that witness wrote, or a scribe later wrote/translated)... same as you would test something that a witness SPOKE.
Peace,
tammy