answer to atheist question, page-46

Religious distress is at the same time the expression of real distress and the protest against real distress. Religion is the sigh of the oppressed creature, the heart of a heartless world, just as it is the spirit of a spiritless situation. It is the opium of the people. The abolition of religion as the illusory happiness of the people is required for their real happiness. The demand to give up the illusion about its condition is the demand to give up a condition which needs illusions.
Karl Marx, Critique of Hegel?s Philosophy of Right
http://atheism.about.com/od/weeklyquotes/a/marx01.htm

I think it was Ceasar who said if you want to control the people give them Religion