The question of authority is a massive one, for those who are Christians and for those who aren't. Where is truth to be found? Whom should I trust? What is 'pure' information, and what is spin?
The question is particularly acute when it comes to the Christian claim to know about God. It is not that claiming to know about God per se is particularly problematic as a concept: many people of many different religions and philosophies make the claim. The problem is that the Christian stubbornly refuses to base their knowledge of God on anything other than a very old book. Indeed, they base their claim to know that God exists on this self-same book. Most problematic, they base their claim to that God can be known through this book on this book. (Still following?) The more philosophically inclined will notice at once that this is a circular argument, so called because it goes in circles:
"why should I believe in God?"
"the Bible says so."
"why should I believe what the Bible says?"
"because the Bible come from God."
"why should I believe in God?"
...and so on and so on, ad infinitum, or at the very least ad boredom.
The thing is, this is the right thing for the Christian to say. As soon as they give any other reason for believing the Bible than "the Bible comes from God", they launch themselves on to a sea of uncertainty and relativity.
What answers, after all, might they give?
Perhaps that the Bible changes lives? But demonstrably so do many books for which much less exalted claims are made.
Perhaps that the Bible is very old? Indeed. But so is Homer, and no-one shapes their lives by him. (I hope).
Perhaps that archaeological evidence backs up the Bible? Maybe it does, but it has to be said that there is no archaelogical evidence for anything directly supernatural, and so this line of argument would most naturally lead us to say there is some truth in the Bible but probably not much.
No, the Christian trusts the Bible because it comes from God, and they know it comes from God because it tells them so.
So I can give lots of reasons why I think you should look at the Bible - some external (like archaeology) and some internal (like the fact that it simply makes sense). But if you look at it seriously, read it carefully, and find that it is not from God, there is nothing I can do except tell you to read it again. If that seems like a dead end, I propose that there is no other way to go.
Like my previous question, this one also comes down to the concept of revelation. If God has revealed himself in history - in the real world of time and space - and if he still reveals himself through the record contained in the Bible, then and only then we might know something about God. If not, then nothing we can do can get us that knowledge - all we will have is guesswork.
Of course, for the Christian this foundational point is not in fact circular. It is circular in terms of its human logic, but it is also the point at which God actually speaks, breaking in to the circle from outside with his powerful word, which grips us and compels us to hear him. And thence comes certainty, and from nowhere else.