That's actually quite a hard question.  The scriptures or writings in the bible are considered by most believers to be the words that God Himself wants us to have as our manual for life.  Who wrote them or selected them for inclusion in the bible is relatively unimportant if we believe that they are insppired by God.  Proving that in a scientific way is almost impossible but I and millions of believers worldwide can confirm that scripture simply works when you treat it as being God's direct word for us, believe it and act accordingly.  My own experience is that the more I simply trust that this book is true, the more it hangs together.  When I read, believe and act upon the truths in it, they suddenly become true for me.

We have also observed that for most Christian believers, the scripture is the most common way in which God speaks to us.  Let me give you an example - a few weeks before I wrote this, my wife was found to have a serious illness.  Many people gave me advice, encouragement etc.  On the next Sunday I preached about trusting God and used a verse from Romans which says that " ... we know that God causes everything to work together for the good of those who love God and are called according to His purpose for them." (Romans 8:28).

A few days later I was flicking through a video of the meeting that we had taken for training purposes.  I happened to play the bit where I read that verse and, as I heard it again, my spirit just came alive and I knew that God was 'giving' me that verse.  It was imprinted on my mind and it has repeatedly come back to me since.  That's just an example of how God speaks to us through His Word.  Of course, we have to be willing to listen!

It is frequently mentioned that Jesus and the apostles taught God's Word and we believers are encouraged to go out and teach the Word of God all through the New Testament.  If we use the bible as our foundation for that, we will never be disappointed!