The easy answer would be to attribute it to Feuerbach, but as with most easy answers—when it comes to Marx—this answer is too easy. Let us start with the famous quote in full:
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, the soul of soulless conditions. It is the opium of the people.^1
This is the young Marx—while I do not agree with Althusser's "epistemological break" in Marx, I do not reject that Marx, as all people do, developed his thought over time, and that there was at least some development in his interests and his positions—undoubtedly lying sleepless in his bed, with his worry over the human spirit and its alienation. But the same young Marx doesn't reject religion in this famous passage, as many interpreters have presented it as. Quite contrary, our young Rhenish friend presents here a quite understanding image of religion. "The sigh of the oppressed creature" is the human essence, longing for a freedom that does not exist in Earthly terms. It is false, that much Marx still maintains. But it isn't condemnation of religion we see here, religion is "bad" per se to the young Marx. What Marx actually condemns here, but more clearly in the Thesis on Feuerbach later, is the simple criticism of religion, and not religion on its own. Marx actually writes about this, in the passage just above the more famous above quoted passage:
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 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 universal basis of consolation and justification. It is the fantastical 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.^2
So, Marx doesn't say that religion is good, quite contrary. Marx says that religion is the opium of the people. It is used to dull the harshness of capitalist society. It is, to use another Marxist term, ideology. As the opium of the people, it fits right into another of Marx's famous images, ideology as the cloud of smoke, obscuring the real world. Marx says that religion is a product of society, it is born out of the conditions of the world, and not out of man's mind alone. It isn't a personal delusion, but rather, it is a part of society, it is a social thing, a social construct if you will. Religion is the pie in the sky, quite literally, as the people do not have pie here on Earth, but are bound in thrall and squallier. But let us move on in our text, past the famous passage, and see what our Moor has in store:
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 conditions 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.^3
I bet you can see where Marx is going now, right? Marx isn't actually about that criticism of religion. He is about the "ruthless criticism of all that exists." And the young Marx, wonderfully dialectically, extracts from the criticism of religion, in a still visibly Feuerbachian way, the criticism of law, the state, society as it is. And as Marx further goes on to say in his thesis IV:
Feuerbach starts out from the fact of religious self-alienation, of the duplication of the world into a religious world and a secular one. His work consists in resolving the religious world into its secular basis.
But that the secular basis detaches itself from itself and establishes itself as an independent realm in the clouds can only be explained by the cleavages and self-contradictions within this secular basis. The latter must, therefore, in itself be both understood in its contradiction and revolutionized in practice. Thus, for instance, after the earthly family is discovered to be the secret of the holy family, the former must then itself be destroyed in theory and in practice.^4
Ofc. it shouldn't be read as Marx being uncritical of religion, or Marx thinking religion was fine and dandy. Marx saw religion as ideology, a way to perpetuate the powers that be, the world's "consolation and justification." The root of Marx's criticism of religion hence isn't Feuerbach and his criticism of religion as alienating, but rather religion as symptom of an alienating world. As a part of the justification of the subordination of labour, as a hurdle to the emancipation of mankind through the emancipation of the proletariat.
Marx thus rejected religion as such, but from this should not be extrapolated a rejection of the spiritual. We know from Marx's amazing style of writing, that texts like the Bible and figures from Jewish mythology were no strangers to him, and he gladly, even as the "older" Marx, used references to folklore and mythology. But also on the personal spiritual level, in the text we examined above, Marx is very explicit, that the criticism of religion, and of the world that it is a product of, isn't just to rid man of whatever happiness that they may derive from their religion, but rather, to create a society where man can be the focus of man:
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. The criticism of religion disillusions man, so that he will think, act, and fashion his reality like a man who has discarded his illusions and regained his senses, so that he will move around himself as his own true Sun. Religion is only the illusory Sun which revolves around man as long as he does not revolve around himself.^5
What we often hear about Marx, this great emancipatory thinker, isn't always the truth, and when it is the truth, such as in this short quote, it is seldom the whole truth. I hope to have, with my short comment, shown how a near study of Marx's texts, may show the preconceived notions about Marx are wrong. Working with the very text we know the quote from, we get a completely different picture of what it means, from the general preconceived notion of the anti-religious Marx. Marx was anti-ideology, of which religion happens to be part. But he does not reject spirituality, and hence religion in its broadest sense.
1: Marx, Karl. "Critique of Hegel's Philosophy of Right. Introduction" in Early Writings. Penguin Books. 1975. p. 244
2: Ibid.
3: Ibid.
4: Marx, Karl. Thesis on Feuerbach. marxists.org. 1845
5: Marx, Karl. "Critique of Hegel's Philosophy of Right. Introduction" in Early Writings. Penguin Books. 1975. p. 244