fanaticism

So in the book I'm reading for my English class, Zen and the Art of Motorcycle Maintenance by Robert Pirsig, there's an interesting quote:

"No one is fanatically shouting that the sun is going to rise tomorrow.They know it's going to rise tomorrow. When people are fanatically dedicated to political or religious faiths or any other kinds of dogmas or goals, it's always because these dogmas or goals are in doubt."

I thought it kind of related to some of the people that Donald Millertalks about, the ones that he says go about sharing their faith by preaching at people instead of just being there. Who are they trying to convince? The "unconverted" or themselves?