As I just said over in the other thread:
Fundamentalism is essentially an absolutist and exclusivist position, combined with an aggressive tendency to proselytize that position, and to deride the positions of others.
In other words, a belief that one is absolutely right, that all other positions are wrong, and an aggressive willingness to try and change other people's minds to conform with one's own "right" belief, and to insult and demean those who do not agree.
This attitude is found in religious extremists and in atheist extremists. It is not about belief in God, but about extremism.