[Quip: But if you compare the social behaviors of the loudest self-professed atheists and the comparable theists...]
I was thinking that Marxism-Leninism was the Official Ideology of the U.S.S.R. under Stalin. Atheism was merely one of M-Lism's (many) points of doctrine - probably buried a fair number of slides down in the PowerPoint deck.
"Religion is the sigh of the oppressed creature, the heart of a heartless world, and the soul of soulless conditions. It is the opium of the people.
The complete quote shows that Marx sees religion as playing an ambiguous role. For him religion is often used to preach resignation "it doesn't matter what you suffer in this world, the next one is good". But sometimes it's at the center of the rebellion of the "oppressed creature". Think the black civil right movement in the US or the Liberation Theology in South America. So it's not blanket hostility.