When Karl Marx stated that religion was the opiate of the masses , he was referring to the way that religion kept workers obedient by promising happiness in the afterlife.
Karl Marx felt very strong about organized religion. He felt that that religion was used as a tool to keep citizens obedient. Organized religion would help ensure that no citizen would question the society they live in. Marx considered religion as a technique by powerful officials to keep normal citizens clueless as to what is going on in a society.