But under no circumstances ought we to fall into the error of posing the religious question in an abstract, idealistic fashion, as an “intellectual” question unconnected with the class struggle, as is not infrequently done by the radical-democrats from among the bourgeoisie. It would be stupid to think that, in a society based on the endless oppression and coarsening of the worker masses, religious prejudices could be dispelled by purely propaganda methods. It would be bourgeois narrow-mindedness to forget that the yoke of religion that weighs upon mankind is merely a product and reflection of the economic yoke within society. No number of pamphlets and no amount of preaching can enlighten the proletariat, if it is not enlightened by its own struggle against the dark forces of capitalism. Unity in this really revolutionary struggle of the oppressed class for the creation of a paradise on earth is more important to us than unity of proletarian opinion on paradise in heaven.
That is the reason why we do not and should not set forth our atheism in our Programme; that is why we do not and should not prohibit proletarians who still retain vestiges of their old prejudices from associating themselves with our Party. We shall always preach the scientific world-outlook, and it is essential for us to combat the inconsistency of various “Christians”. But that does not mean in the least that the religious question ought to be advanced to first place, where it does not belong at all; nor does it mean that we should allow the forces of the really revolutionary economic and political struggle to be split up on account of third-rate opinions or senseless ideas, rapidly losing all political importance, rapidly being swept out as rubbish by the very course of economic development.
https://www.marxists.org/archive/lenin/works/1905/dec/03.htm