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"Religion is the opium of the people" is one of the most frequently paraphrased statements of Karl Marx. It was translated from the German original, "Die Religion ... ist das Opium des Volkes" and is often referred to as "religion is the opiate of the masses." The quotation originates from the introduction of his proposed work A Contribution to the Critique of Hegel's Philosophy of Right; this work was never written, but the introduction (written in 1843) was published in 1844 in Marx's own journal Deutsch-Französische Jahrbücher, a collaboration with Arnold Ruge. The phrase "This opium you feed your people" appeared in 1797 in Marquis de Sade's text L'Histoire de Juliette and Novalis's "[R]eligion acts merely as an opiate" around the same time. The full quote from Karl Marx is: "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 quotation, in context, reads as follows (emphasis added):
The foundation of irreligious criticism is: Man makes religion, religion does not make man. 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 encyclopaedic 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.
Criticism has plucked the imaginary flowers on the chain not in order that man shall continue to bear that chain without fantasy or consolation, but so that he shall throw off the chain and pluck the living flower.
In the Marquis de Sade's Juliette, published in 1797 (trans. Austryn Wainhouse, 1968), Sade uses the term in a scene where Juliette explains to King Ferdinand the consequences of his policies:
Though nature lavishes much upon your people, their circumstances are strait. But this is not the effect of their laziness; this general paralysis has its source in your policy which, from maintaining the people in dependence, shuts them out from wealth; their ills are thus rendered beyond remedy, and the political state is in a situation no less grave than the civil government, since it must seek its strength in its very weakness. Your apprehension, Ferdinand, lest someone discover the things I have been telling you leads you to exile arts and talents from your realm. You fear the powerful eye of genius, that is why you encourage ignorance. This opium you feed your people, so that, drugged, they do not feel their hurts, inflicted by you. And that is why where you reign no establishments are to be found giving great men to the homeland; the rewards due knowledge are unknown here, and as there is neither honor nor profit in being wise, nobody seeks after wisdom.
I have studied your civil laws, they are good, but poorly enforced, and as a result they sink into ever further decay. And the consequences thereof? A man prefers to live amidst their corruption rather than plead for their reform, because he fears, and with reason, that this reform will engender infinitely more abuses than it will do away with; things are left as they are. Nevertheless, everything goes askew and awry and as a career in government has no more attractions than one in the arts, nobody involves himself in public affairs; and for all this compensation is offered in the form of luxury, of frivolity, of entertainments. So it is that among you a taste for trivial things replaces a taste for great ones, that the time which ought to be devoted to the latter is frittered away on futilities, and that you will be subjugated sooner or later and again and again by any foe who bothers to make the effort.
Ihre sogenannte Religion wirkt bloß wie ein Opiat: reizend, betäubend, Schmerzen aus Schwäche stillend.
Their so-called religion acts merely as an opiate: irritating, numbing, calming their pain out of weakness.
We have used the Bible as if it were a mere special constable's hand book, an opium dose for keeping beasts of burden patient while they were being overloaded, a mere book to keep the poor in order. 
Religion is one of the forms of spiritual oppression which everywhere weighs down heavily upon the masses of the people, over burdened by their perpetual work for others, by want and isolation. Impotence of the exploited classes in their struggle against the exploiters just as inevitably gives rise to the belief in a better life after death as impotence of the savage in his battle with nature gives rise to belief in gods, devils, miracles, and the like. Those who toil and live in want all their lives are taught by religion to be submissive and patient while here on earth, and to take comfort in the hope of a heavenly reward. But those who live by the labour of others are taught by religion to practise charity while on earth, thus offering them a very cheap way of justifying their entire existence as exploiters and selling them at a moderate price tickets to well-being in heaven. Religion is opium for the people. Religion is a sort of spiritual booze, in which the slaves of capital drown their human image, their demand for a life more or less worthy of man.