|The oldest Hebrew reference to the city of Jerusalem, outside the Bible|
It's one of those questions... It has many different answers.
In this post, just one answer (the others will come): it's there because people learnt how to write things down, and while it's likely that the stories that are told in the Bible were first told by word of mouth, eventually someone decided to write them down. That was no small thing in the ancient world. Only really important things got written down. You didn't write your shopping list on a piece of papyrus. Unless you were the king, in which case your shopping list was probably pretty long and you had plenty of papyrus.
When someone tells me something important, I pull out my phone and write it down. It might be something I need to do, or a book recommendation, or a future meeting. I know that I'll forget whatever it is unless I write it down. That wasn't how writing worked in the ancient world - people were trained to memorise that kind of stuff, so they wouldn't have to write it down. This means that when people told stories about Jesus, we can have quite a high level of confidence that people were able to pass on the right information from person to person. From the little we know, it would appear that the teaching of Jesus, the miracle stories and the week leading up to Jesus's execution were all memorised very faithfully. For example, when the gospel writers Matthew, Mark and Luke tell the same story, they sometimes change little details, but the words of Jesus are treated with a high degree of reverence.
By the time these stories got written down there had already been a great deal of arguing and editing done. These were the stories that (probably) many different churches were telling about Jesus. Writing them down gave them a permanence that we can't feel now. If I make a spelling mistake, it's no big deal. In fact, if I don't like what I've written I can just press backspace on my computer and start again. But writing things down in the ancient world was significant: we're going to tell this story and not that one.
Christians say that one of the most important things about their faith is the belief that Jesus was both human and divine at the same time. Jesus wasn't God pretending to be human, he really was human. And God too. The Bible is similar. It's not a divine book that fell from heaven and is just pretending to be a human creation, it really is a human creation that also reveals God in a special way. When we treat the Bible as untouchable we deny God's amazing partnership with humankind.
In the next big post I'm going to talk about how all the different parts of the Bible got put together, how they were chosen and how we got a thing called the Bible, but before that I want to go off piste for a quick rant about something the Bible isn't.