There is no fundamentalist atheist. That's religious propaganda. Non-belief is not a fundamentalist belief. You don't believe in atheism because atheism is non-belief.
There is an insidious movement to condemn atheists by calling them "fundamentalists".
This is not only a fabrication but suggests the intolerance of the religious community. There are no atheists that would call themselves "fundamentalists" but there are plenty of Christians who would apply that term to themselves. The Christians that use the term "fundamentalist" to describe atheism are enabling the Christian fundamentalists in their persecution of non-believers. After Christian homophobia comes atheophobia.

Frank Hamilton