Religion is the opium of the people
This is probably the best-known quotation by Karl Marx, the German economist and Communist political philosopher. The origin German text, in Critique of Hegel's Philosophy of Right, 1843 is:
Die Religion... ist das Opium des Volkes
This has been translated variously as 'religion is the opiate of the masses', 'religion is the opium of the masses' and, in a version which German scholars prefer 'religion is the opium of the people'. The context the phrase appears is this:
"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."