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CB: I think Lenin may be taking philosophy a bit more seriously than you are, Justin. It does have to be a major part of the source of revolutionary theory. For Marxists materialist theory must part of the source of inspiration to change the world. Recall that Marx said that previously idealism, which in the Marx/Engels/Lenin vocabulary is somewhat synonymous with religious thinking, had supplied the active subject, and materialisms prior to Marx's had not. In other words for Marxists , philosophy, like the personal for feminists, is political. "Philosophy" has to be some of the source for revolutionary inspiration. When an idea grips the masses it becomes a material force and all that.
Also, a number of the Bolsheviks who took to empirio-criticism were explicitly advocating fideism , and I think "God building". This was in the period of reaction after the 1905 Revolution. So , there may be a sort of turning to religion in the typical sense of a source of succor for the blues that Lenin feels he is combatting. This is a book for Bolsheviks especially.
But also recall, that though Lenin was sharp in this polemic, the Bolsheviks he disagreed with here were not excluded from important government positions. In other words, Lenin had a liberal attitude on this dispute in practice.
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After all, the empiriocritics were, and most empiricists are, Enlightenment agnostics or atheists; Hume of course was a famously savage critic of religion (if Berkeley was a bishop). A number of distinguished empiricists that Lenin would have known about were socialists if not revolutionaries, e.g., J.S. Mill (not that Lenin would have had much patience for Mill's relentlessly bourgeois socialism--does he talk about Mill anywhere? I can't recall that he does).
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CB: I don't think Mill is mentioned in this book. Certainly not featured.
Even if agnostics and atheists might unite at one level, at some level it is important to distinguish atheism from agnosticism. I take the Kant's notion of the unknowability of things-in-themselves while saying that they do exist , to be the "agnosticism", or sort of fence-sitting that Engels and Lenin object to. Also, they are criticizing Kant's idea that logic is in the structure of the brain and not a reflection of material reality, as a concession to idealism.
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Although this was after Lenin's time, I have remarked here that the logical positivists of the Vienna Circle were radicals, revolutionary socialists, and (in Neurath's case) Marxist, with the exception of Schlick.
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CB: Think how sharp were Marx's criticism of Proudhoun and Lassalle (?) , but in practice Marx and Engels worked with followers of these two. Lassalle stayed at Marx's house once. As you say, shouldn't Lenin in the role of philosopher poke holes in anything and everything.