2 Peter 1:20-21 (NIV)
Above all, you must understand that no prophecy of Scripture came about by the prophet's own interpretation. For prophecy never had its origin in the will of man, but men spoke from God as they were carried along by the Holy Spirit.
The Bible is an amazing book. There are sixty-six different books penned by forty different authors over a fifteen hundred year period of time. The amazing thing is that it the message is consistent throughout. Yes there are some difficult passages but when interpreted in light of the many more clear passages the Bible speaks as a single voice.
This verse speaks of prophecy and that word does not mean what it is commonly thought to mean. It means simply the forthtelling the truth of God which includes the truth about the past and the truth about the future. Of course if it's true, it neither changes nor grows old. One thing that this verse makes clear is that these men were not the authors; they were moved to write what they wrote by the Holy Spirit. Did the Holy Spirit put the exact words into their minds? Well, yes but they would been the words and vernacular they were familiar with which gives each book it's characteristic style.