The quote’s been posted, but it’s really more of an observation; it’s a symptom of the state of things, and should be interpreted through that lens. Think of it less as a moral evil and more as the 19th century equivalent of our bullshit Treat Discourse. People saw religion as a path to salvation from their very real, material problems, even though it’s just a way of interpreting our perception of the world. It’s less common today, at least in America. Now we have Elon worship and the rise and grind hustle culture bullshit, and whatever else people fixate on as they yearn to be free of the yoke of oppression in the various forms it takes.

Marx meant that religion provides an outlet for the material problems of the oppressed and thus runs counter to revolutionary action at its core. We know this doesn’t mean religious people can’t be revolutionary, but he did make a good point about it having the capacity to distract or to act as a substitute for demanding material change.

I recently heard a local Muslim religious leader give a speech about Palestine. This particular person decided to focus on “the” meaning of hijab (it of course means different things in different contexts) and to then say that as Muslims, it must be understood that this unbearable suffering in Palestine is only temporary, that it can be accepted because the afterlife will vindicate them.

On one hand, I would not criticize anyone for using religion as a way to heal from oppression. At the same time, you can see how “only the next life matters” could lead someone to place less emphasis on materially fighting against oppression and therefore do a worse job of opposing it.

We of course see this kind of thing in all religions, they provide explanations for injustice that usually place blame outside of what can be materially helped and justice that is entirely outside the scope of our lives. Many have the same basic idea that this life is temporary and doesn’t matter in comparison to the infinite later (whatever that might be).

When it comes to organized religion, one can also see how it becomes merged with the ruling class, becoming an inherent component of social order and control and adapting its beliefs to what is needed by the ruling class. For example, early Christianity was pretty clear about all interest on debts counting as usury. There was no distinction in language whatsoever. But the Roman ruling class depended on debt, so the distinction was invented and suddenly there were good moneylenders and bad moneylenders and the ruling class’s status quo was - you guessed it - good moneylenders!

Anyways, I’m getting off-track. Marx was talking about religion offering a false consciousness that detracts from revolutionary behavior.

I do think I should probably mention something else, though: many communist movements attempted to impose atheism and/or very strict secularism on populations that were not receptive to it and thereby shot themselves in the foot. It’s good to understand Marx’s point about what religion can offer and do but to not take it to a false extreme that deposing religion is the first priority. You’ll end up, ironically, actually feeding organized religion that will redirect revolution away from being proletarian, as the people you should be able to recruit and radicalize will instead go to the alternatives, including religious organizations.

Marx isn’t anti-religion because “lol man in the sky silly”. Marx is anti-religion because it is the product of an inverted world, a class society. Marx believes that, if class society is abolished, if the topsy-turvy (this is the word he uses in e.g. “On ‘The Jewish Question’” or the rest of “Contribution to the Critique of Hegel’s Philosophy of Law”) world is turned upright, religion will wither away into nothing. Marx believes this is only possible when all the social relations that underlie society are laid bare in an understandable form (i.e. with communism), and that it will not disappear until the topsy-turvy material world disappears.

As Marx states in the former essay, Marx doesn’t believe religion can be abolished by decree (and he points out that state secularism is often just reskinned Christianity). In both essays, Marx actually details how the state itself, money itself, are religions; for Marx religion is the work of human mind alienated from humans and dominating them.

In the full quote (which emizeko posted) Marx outright refers to religion as the general theory of this world (i.e. the topsy-turvy world), as an encyclopaedic compendium. Elsewhere (either in Contribution to the Critique or in On the Jewish Question) Marx refers to religion as a register of the theoretical struggles of mankind. Based on Marx’s usage of various bible quotes and themes in his work (even Capital), it seems likely that he treated religion as he would e.g. liberal economists or members of parliament (i.e. with critical analysis and an eye towards useful stuff for his own critique)

anticommunists chop it to a sentence or two. read the quote with more context:

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.

from https://www.marxists.org/archive/marx/works/1843/critique-hpr/intro.htm