Fundamentalism

I'm reading Faith Unraveled by Rachel Held Evans.  Near the very beginning of the book, she writes:

I was a fundamentalist not because of the beliefs I held but because of how I held them: with a death grip.  I would take God himself to finally pry some of them out of my hands.  The problem with fundamentalism is that it can't adapt to change.  When you count each one of your beliefs as absolutely essential, change is never an option.  When change is never an option, you have to hope that the world stays exactly as it is so as not to mess with your view of it.

I can see that viewpoint.  I was listening to the radio on the way into work one day. The story was about a person who believed in a literal interpretation of the Creation story.  He defended it against any assault and believed that if this interpretation was wrong, then his whole faith would be threatened.

I was shocked by that.  How can that be? How can the loss of a seven day creation belief threaten an entire faith system?

And why do we hold so tight to a belief? Do we think that belief is God?