I was surprised at Hunsberger and Altemeyer's findings in Atheists: A Groundbreaking Study of America's Nonbelievers that atheists score high on their "DOGmatism" scale. And then I actually read the questions: Any person—as the authors explicitly note—who is convinced of the truth of some proposition will score high on this scale.
What separates dogmatism and fundamentalism from ordinary conviction as to the truth is not the confidence with which the belief is held, but the rational justification for that belief.
When investigating specifically religious belief, the authors go to considerable length to examine the rational basis of those beliefs. But because of the practical limitations on the Atheists study, only a single question—considerably flawed—addresses the rational basis of atheism. (The question and its analysis are lengthy and beyond the scope of this essay.)
Fundamentalism in actual theistic religion has gained considerable notoriety precisely because it is preposterous to believe any religion is actually true. There may be good reasons for doing so, but religion is really nothing but lies and bullshit believers tell themselves to make themselves feel better—Atheist Eve is spot-on when she likens prayer to masturbation. There's nothing wrong, I suppose, with (mental) masturbation per se, but it's not the truth.
When one is talking about ordinary matters of prosaic truth, there is not much room for ambiguity or difference of opinion. The Earth really is round. Rocks really do fall from the sky. Humans and chimpanzees really do share a common ancestor. Human beings really are making the Earth get warmer. HIV really does cause AIDS and can be passed through semen. These statements are true If you disagree with any of these statements, you're wrong.
Without actually looking at the rational basis for one's belief, any person convinced of some ordinary, prosaic truth will look just as "dogmatic" or "fundamentalist" as any Bible-thumper—because he too believes in the truth of what the Bible says.
That's what true means: That there's a right answer, and all the other answers are wrong. And it is the case that no small few atheists, notably Dawkins and Harris, and I myself, do believe that atheism is in fact the truth.
Of course atheists look like they're dogmatic; just as—if you consider evolution mere opinion—evolutionary biologists seem dogmatic; just as—if you consider global warming mere opinion—Al Gore seems dogmatic; just as—if you consider medicine mere opinion—Physicians seem dogmatic about pushing condoms.
The bottom line is that atheists seem dogmatic and fundamentalist because we really are convinced that atheism is true. What separates us from the dogmatic and fundamentalist religious is that we actually do have the philosophical and empirical arguments to back up our positions; All the religious have is epistemic nihilism; in plain English: Lies and bullshit.
I will invoke Ingersoll:
We have heard talk enough. We have listened to all the drowsy, idealess, vapid sermons that we wish to hear. We have read your Bible and the works of your best minds. We have heard your prayers, your solemn groans and your reverential amens. All these amount to less than nothing. We want one fact. We beg at the doors of your churches for just one little fact. We pass our hats along your pews and under your pulpits and implore you for just one fact. We know all about your mouldy wonders and your stale miracles. We want a this year's fact. We ask only one. Give us one fact for charity. Your miracles are too ancient. The witnesses have been dead for nearly two thousand years.
Show me that one fact: If then I still retain my conviction, you may with justice call me a dogmatist and fundamentalist.