Absolute faith in anything is bad.  When people put absolute faith in something and a question arises that challenges that faith, people become irrational and immoral in an attempt to squash that challenge.  This leads not only to religious fundamentalism, but also political fundamentalism.

The most obvious example is Islamic fundamentalism, which has given itself the mission of destroying every person on Earth that questions its faith.  Another example would be Russian communism.  They had so much faith that their leadership would lead to Marx's promised utopia, that when it didn't happen, they turned to violence and massacre against the people to make it happen.

I can't think of any accomplishment in human history that could not have been accomplished without absolute faith. You don't need to have complete doubt to have some doubt, and you can still have faith, even with some doubt.  That is the basis for reason