Marx was a Jew, and for political reasons his family and he became Lutherans. (It was the state religion of Prussia, where he grew up.)
In later years, Marx claimed:
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.
Marx was on the East German 100 mark note.
(I have to confess that I shamelessly stole the "Killer Fact" theme from the inimitable Harry Hutton's Chase Me Ladies, I'm In The Cavalry. Hutton also runs killer-fact.com, a Website dedicated to cataloging his Killer Facts, giving silly quizzes, etc. Highly recommended!)