Karl Marx's quote "Die Religion ... ist das Opium des Volkes"-- translated as "religion is the opiate of the masses" comes from the introduction of his work "Contribution to Critique of Hegel's Philosohpy of Right".
In saying this, Marx is pointing out that religion is an excellent mechanism for appeasing and gratifying a populace without actually doing anything for them. Opium addiction was rampant in his time. Opium users were generally regarded as hiding from the world in a drug-induced stupor. The drug puts users into a euphoric state regardless of their actual situation in life.
Religion can be used, and has been used many times in history, by the ruling class to calm and pacify a people who otherwise may have every reason to be rebellious and militant.
Karl Marx used this reasoning as the basis for the atheism which is so prevelant in his ideals.
The quote is often used outside of Marxism as an argument for secular government in general.
The following is a larger quote which puts our subject into context.
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Religion is, indeed, the self-consciousness and self-esteem of man who has either not yet won through to himself, or has already lost himself again. But man is no abstract being squatting outside the world. Man is the world of man-state, society. This state and this society produce religion, which is an inverted consciousness of the world, because they are an inverted world. Religion is the general theory of this world, its encyclopedic compendium, its logic in popular form, its spiritual point d'honneur, its enthusiasm, its moral sanction, its solemn complement, and its universal basis of consolation and justification. It is the fantastic realization of the human essence since the human essence has not acquired any true reality. The struggle against religion is, therefore, indirectly the struggle against that world whose spiritual aroma is religion. Religious suffering is, at one and the same time, the expression of real suffering and a protest against real suffering. 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 abolition of religion as the illusory happiness of the people is the demand for their real happiness. To call on them to give up their illusions about their condition is to call on them to give up a condition that requires illusions. The criticism of religion is, therefore, in embryo, the criticism of that vale of tears of which religion is the halo. -- Karl Marx
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