Fundamentalist atheism is defined as a rigid, intolerant, and dogmatic adherence to atheism or an atheistic ideology. The theory behind this definition is that there exists a fundamentalism which is atheistic and which atheists adhere to much like some Christians adhere their own fundamentalist Christianity. Fundamentalist atheism tends to be used interchangeably with militant atheism, new atheism, and antitheism.
The label "fundamentalist atheism" is used pejoratively as a criticism of contemporary atheism by associating it with fundamentalist ideologies which are intolerant, militant, oppressive, and anti-democratic. Critics of atheism only employ the label fundamentalist atheism as a means for discrediting atheism, not as a way to provide an objective, neutral description of some phenomenon.
This is demonstrated by the fact that you need some sort of ideology in order to be fundamentalist and atheism — even when defined narrowly as denying the existence of gods — is at most a single belief, not an ideology. If atheism by itself cannot be an ideology, then it cannot possibly be fundamentalist no matter what sort of attitude an individual atheist may have.
"Passion for passion, an evangelical Christian and I may be evenly matched. But we are not equally fundamentalist. The true scientist, however passionately he may 'believe', in evolution for example, knows exactly what would change his mind: evidence! The fundamentalist knows that nothing will."
- Richard Dawkins, "How dare you call me a fundamentalist"
There is, however, a need to remember that militant or fundamentalist atheism, which seeks to overturn religious belief by force, is as dangerous as any other form of fundamentalism. Atheism's most authentic political expression thus takes the form of state secularism, not state atheism.
- Julian Baggini, Atheism: A Very Short Introduction
In the broad version of atheism, people simply do not accept the basic premise of theism; in the narrower and more determined position, they believe that the theistic position is not only misguided but actively wrong. Sometimes this is called 'fundamentalist atheism'. (The concepts of fundamentalism and atheism should not really be mixed but critics and theistic apologists like to label the extreme end of atheism as 'fundamental'....)
- Nick Harding, How to Be a Good Atheist
