Just as men were inspired to write Gods word, others were inspired to determine what writings that "claimed" to be the word of God were and were not. Even Paul, in one place clarifies that it is not God talking but him:
1 Corinthians 7:12
To the rest I say this (I, not the Lord): If any brother has a wife who is not a believer and she is willing to live with him, he must not divorce her.
Meanwhile, the wisdom of the Bible has proven itself. The greatest works of man have shown their flaws over the decades and centuries, yet such a work as this has withstood the test of time even with the discoveries of the modern age, where culture and technology change as much in a decade as they did in several generations during most of human history.
And the documentation is there. A staggering amount, considering the age of the scriptures. And as the earth and solar system (and universe) give up their secrets, the Bible continues to be confirmed over and over again.
But enough about that. It is the personal relationship with Christ that actually confirms it, and that cannot be proven any more than someone could drop me in the middle of iceland and expect to prove to the people there that my wife exists.
But then, to have a personal relationship with her, I need not prove anything to them, unless they are the ones springing for my plane ticket back to her. 8^>
I got it. At one time, some men wrote the word of God but later some other men determined that those men lied and those parts of the Bible were not the word of God.