It depends. There are "fundamentalist Marxists" who go back to the original texts of Marx and Engels, and claim that true communism as envisioned by Marx has never actually happened (a lie, of course). There are atheists who think the very word of Noam Chomsky is gospel. And there are secular humanists who seemingly take every word of the Humanist Manifesto as gospel as well (and, hypocritically, are usually the first to accuse capitalists of 'market fundamentalism', when they place unquestioning faith in the perfectibility of people and governments).
These share all of the same characteristics of theistic fundamentalism-blind, unquestioning faith, based on the literal interpretation of ideas instead of any basis in fact, and yet vague enough to resist being refuted in the face of overwhelming evidence. Now do you all see where I was coming from when I said all forms of fundamentalism are the same?