I'm not sure if this question should even be considered as it ties very closely to Does God Exist?
And the question itself feels vague, even to me. But I felt I might as well put this question in because I know that some people do doubt the validity of the Bible.
I apologize in advance if my words on this topic sound like mindless rambling.
First, let me speak on the basics about the Bible.
Unlike other religions of the world, the Holy Bible was written by many authors. I consider this very telling about the uniqueness of the book. For example, anyone can start a religion. And anyone could write up their own book about their religion. But then... can we really trust the author? Especially if it's from a long time ago, we can't confirm that what he/she has written is true by ourselves.
One man could have just written a ton of crazy things, told everyone to believe in it, and there we go... RELIGION! Who can confirm or deny? Who can bear witness?
But thankfully, the Bible is different. I can't remember off the top of my head how many authors there are, but I know there are at least 10, but there are definitely A LOT. Does that make the Bible possibly less valid, or more valid? Well, that depends on what you believe the Bible is supposed to be.
What is the Holy Bible supposed to be?
We, as Christians, believe that the Bible is the WORD OF GOD. Note that I did not say the words of men, for obvious reasons. If God exists, is a spiritual being, and wants to communicate with us with words, how are the ways God could go about it?
1) Poof a book into existence.
2) Speak to people and have them write it
3) Speak to one person and have him/her write it
What's the problem with poofing a book into existence? Well, there isn't a problem with it, as there is a problem with proving that it appeared out of thin air. If someone found a mysterious book that popped into existence, how would he prove that he didn't write it? Then it becomes pointless. Whether or not he wrote it does not matter, because he wouldn't be able to prove anything in anyway. This becomes equal to point 3, because both rely on one person's testimony, which we know is hard to accept.
That is why it is nice that the Bible was written by many authors, over a long period of time, such that many authors weren't even living within the same time periods.
Yes, it does not prove that the Bible is trustworthy, but at least it doesn't have the issue of "Okay, this is one man's doing." Nope, the Bible is a group effort.
And that at least is consistent with what we believe: That God exists and is eternal. Because of this, he could surely speak to certain people at certain times (throughout human history) and give them certain words to be jotted down.
If you want to look for proof that the Bible can be trusted, you can go search for it. This is really not a topic that I'd like to go on about. Why?
I'll just say it simply:
Is the Bible completely true? That depends solely on your belief of God.
If you can believe that there is an all powerful, invisible God in control of the universe, and that he cares enough about us to try to communicate with us through an understandable language, then you can believe that this God would, without a doubt, use his power to ensure that the Holy Bible given to us is one that we can trust.
Although there is proof of the Bible's consistency if you're willing to look for it: Look up Dead Sea Scrolls if you want to get started. It is physical proof that the Bible has not changed in two thousand years.