There is a big difference between believing something and knowing it to be true through your own experience. The fundamentalism at work here is taking something as true without evidence and then rejecting all other points of view before actually knowing your own to be true. If you know with 100% certainty that X is true then you can reject Y but only if Y is the opposite of X. But, if you have no evidence of X (for example the non existence of God) How can you fully reject Y ( The existence of God) ?