At the blog of Sam Harris, Steven Pinker points out that there is no legitimate frame of reference for characterizing murderous regimes to be "atheist," despite their lack of religiosity:

First, the premise that Nazism and Communism were “atheist” ideologies makes sense only within a religiocentric worldview that divides political systems into those that are based on Judaeo-Christian ideology and those that are not. In fact, 20th-century totalitarian movements were no more defined by a rejection of Judaeo-Christianity than they were defined by a rejection of astrology, alchemy, Confucianism, Scientology, or any of hundreds of other belief systems. They were based on the ideas of Hitler and Marx, not David Hume and Bertrand Russell, and the horrors they inflicted are no more a vindication of Judeao-Christianity than they are of astrology or alchemy or Scientology. Second, Nazism and Fascism were not atheistic in the first place. Hitler thought he was carrying out a divine plan. Nazism received extensive support from many German churches, and no opposition from the Vatican. Fascism happily coexisted with Catholicism in Spain, Italy, Portugal, and Croatia.