How people define what is the "word of God" can be different things, one of those things is believing it literally as a book of dictation from a deity, without any human error or reflective of anything less than accurate scientific and historical truth. That is actually a very modern view of the Bible held by modern fundamentalists. To others "the Word of God" does not mean the Bible, but something quite different than that.
That is an excellent question. Why were people Christians before there was a Bible? We can start with that question as my answer. Why do you think? There was no "Bible" until a couple hundreds years after people were calling themselves Christians.
People became Christians because the message spoke to them on some level or another. Not because there was a Bible to believe in. One can recognize the Bible as just a compilation of various spiritual writings which have meaning to them, without needing to view those writings as supernatural in origin. Human beings have all manner of spiritual insights and can express them through many means, including art and poetry. You don't have to worship a poem or a piece of art and "believe it it" in order for it to speak to you.
There's a great deal more that can be said about this, but that is a quick and easy example to begin with.
As I just explained the "Word of God" can mean something other than "The Bible". In John 1:1 it translates Logos as "Word" but obviously since it identifies Logos with the person of Jesus in verse 14, it cannot, and is not referring to the Bible. "The Bible became flesh and dwelt among us"? Clearly those who conflate the "Word of God" with the Bible have something wrong with their heads.
But to answer your question about the Truth changing. If you used Truth with a capital T, then that is referring to the Divine itself, and not a propositional truth that one "believes in". The Divine is the Ground of all Being and "technically" does not change, because change is a measure of differences over time. God is "eternal", not in the sense of an infinitely long timeline, but rather exists outside of time, and is equally present within every changing moment of time according to our perceptions of it.
What does change is our ideas, views, and understandings over time. How people think of God can and does change over the course of our own lives, from childhood on through our spiritual growth and development. Obviously, if we are changing, then how we view God will change with that.
Sure it's truth, from the perspectives of the people writing it at that time. How we understand the truths they expressed, can and should be held in ways that make sense to us today. Obviously understanding what we do from science about the age of the earth for instance, it would be a great error to assume the writers of the the texts back then had magical knowledge like that, and we should therefore ignore science in favor of a primitive view of the natural world. But this does not therefore mean it invalidates timeless truths such as "love your neighbor as yourself". That is a valid understanding which transcends time.
Of course they are the words of people. Do you think the Apostle Paul was a space alien?
To be spiritual truth, is really a matter of quality. A poet or a musician can speak these timeless truths of God as well. You have this idea in your head that these things are somehow magical or supernatural. That is like how a child imagines these things, like there being a Santa Claus. Because Santa is a myth, that doesn't mean the truth of the
joy of Christmas time is not in fact totally real. Does it?
BTW, it's not your fault that magical view of these things is stuck in your thinking. It's how you were taught by those who think in those terms. But not all Christians think like a child in those ways, such as you find in such statements as this:
Chicago Statement on Biblical Inerrancy - Wikipedia
Because you don't understand any other view of the Bible than that expressed by the fundamentalist point of view? Not all Christians think like they do. You can still find truth, value, and meaning in the words because they are as true today as then. However that does not translate into everything they thought and wrote is to be taken as factual. That's a bad bit of reasoning to think that way. I am more than free to say I disagree with some of those author's points of view in various places. What speaks to the heart as true, has value. It doesn't have to be defended by some erroneously defined statement on so-called "Biblical Inerrancy" as in the Chicago Statement referenced above.
No, not a work of fiction. They also aren't a romance novel either.
Something can be inspired, without needing to be factually correct. Even a work of fiction can be inspired, if it speaks to the human soul. That statement right there, is something that makes a fundamentalists head explode. Are you able to make sense of that statement yourself that I just highlighted?
Does something need to be factually correct, scientifically valid, in order for truth and meaning to be found in it? Do you really believe that, like a fundamentalist believes the Bible must be factual in every way or it has no meaning either? Understanding this, will answer your question how people can believe in God or find truth and value in various spiritual texts without needing to think in the terms that fundamentalists do.
When I said "yes", I was focusing on the fact I believe in God, not the bit about the Bible being a fiction, per se. From a literary point of view it's not considered a work of fiction. But there are in fact "made up stories" within it which are used to convey higher or deeper spiritual truths. This is the nature of what mythologies are and do. The "facts of the story" are not the truth of them. What the made up characters are saying, is the point of them. This is what will maybe help you to begin to separate out the meaning of the symbols from the symbols themselves.
But yes, having experience of the reality of God (which I first had before I was exposed to religious teachings, so it was therefore not based on the thoughts of others), lets me hear when others speak truths that resonate with that experience. It doesn't matter which religion they are speaking it from in the words and symbols they use. It's heard with the ears of the heart, not through rational deductive reasoning processes. Love is not a rational proposition. You don't know it's Love when you encounter it in your life, as the end result of a logical debate with your mind.
I can hear and see the Word of God in the song of bird in the tree in my backyard, or the eyes of a child looking across time to my eyes seeing theirs. I can hear God because I experience God. You don't need a Bible for that. The "Word of God" is all of creation. Creation is God speaking.