For the most part, I think we Christians take for granted the authority of scripture. For most of us, we say, "God said it, I believe it."
But for those outside the Church (and even some inside), it is not so clear cut. We can't assume that everyone really respects the Word of God. It's hard for many to believe the Bible hasn't been corrupted over many years, by many translations, etc.
Unfortunately, people visit our churches each week hearing us believe that no one would dare question the authenticity, origin, or authority of the Bible. From that vantage point, it would really be difficult to believe much about what we say. (On the other hand, I do believe much can be said about the changes Christ makes within us. Our lives are the true evidence of God's truth.)
With all this said, there is a pretty good paper on the origin and reliability of the Word of God. Mark Driscoll's church is changing over to the English Standard Version (ESV) of the Bible and he explains why in this post (and the associated PDF). Download it and check it out!