A famous quote that I have seen attributed to Karl Marx over the years is that 'religion is the opiate of the masses'. I have used the quote myself to bolster religion-as-placebo theories and even theories that religion affects the brain much as endogenous opioids do. Well, in fact, even allowing for liberties in translation, Marx never quite said that. What he did say was:
‘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, it 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.’
Marx was important in trying to interpret religion objectively and in trying to understand its role in society. He believed that men make religion as a 'fantastic realisation of the human essence' rather than the reverse. That seems accurate to me.
The quote comes from Karl Marx, Contribution to the Critique of Hegel's Philosophy of Right, February, 1844