Political Memes

He didn't say that as a bad thing.

The Communist Manifesto spends time talking about how compatible early Christian teachings are with Marx's goals.

As with the "abolition of private property" people quotes what's convenient. The whole quote goes:

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.

To be fair a lot of the quotes attributed to Marx were just common phrases he rephrased, or a giant wall of text reduced by someone else to a memorable quote stripped of all 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.

https://en.m.wikipedia.org/wiki/Opium_of_the_people#Full_quotation_and_history

And it came from an unpublished response to someone else's work. After he was dead his follower pared all of that down to:

Religion is the opium of the masses

And let everyone who heard it, believe it meant whatever they think it means.

Always be wary of authoritarians that are given credit for "speaking plainly" but aren't actually saying anything. Being vague and letting the person interpret it however they want us basic grifting. All that matters is you're gaining their confidence.