“Religion is the opium of the people—this dictum by Marx is the corner-stone of the whole Marxist outlook on religion. Marxism has always regarded all modern religions and churches, and each and every religious organisation, as instruments of bourgeois reaction that serve to defend exploitation and to befuddle the working class.”

– V.I. Lenin, “The Attitude of the Workers’ Party Towards Religion”

“[…] 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.”

– V.I. Lenin, “Socialism and Religion”

“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.”

– V.I. Lenin, “Socialism and Religion”

“[Engels polemicized against those who] gave prominence to religious divisions rather than political divisions, and diverted the attention of some sections of the working class and of the other democratic elements away from the urgent tasks of the class and revolutionary struggle to the most superficial and false bourgeois anti-clericalism. Accusing the would-be ultra-revolutionary Dühring of wanting to repeat Bismarck’s folly in another form, Engels insisted that the workers’ party should have the ability to work patiently at the task of organising and educating the proletariat, which would lead to the dying out of religion, and not throw itself into the gamble of a political war on religion.”

– V.I. Lenin, “The Attitude of the Workers’ Party Towards Religion”

“[…] Engels frequently condemned the efforts of people who desired to be “more left” or “more revolutionary” than the Social-Democrats, to introduce into the programme of the workers’ party an explicit proclamation of atheism, in the sense of declaring war on religion. Commenting in 1874 on the famous manifesto of the Blanquist fugitive Communards who were living in exile in London, Engels called their vociferous proclamation of war on religion a piece of stupidity, and stated that such a declaration of war was the best way to revive interest in religion and to prevent it from really dying out. Engels blamed the Blanquists for being unable to understand that only the class struggle of the working masses could, by comprehensively drawing the widest strata of the proletariat into conscious and revolutionary social practice, really free the oppressed masses from the yoke of religion, whereas to proclaim that war on religion was a political task of the workers’ party was just anarchistic phrase-mongering.

Religion must be declared a private affair. In these words socialists usually express their attitude towards religion. But the meaning of these words should be accurately defined to prevent any misunderstanding. We demand that religion be held a private affair so far as the state is concerned. But by no means can we consider religion a private affair so far as our Party is concerned. Religion must be of no concern to the state, and religious societies must have no connection with governmental authority. Everyone must be absolutely free to profess any religion he pleases, or no religion whatever, i.e., to be an atheist, which every socialist is, as a rule. Discrimination among citizens on account of their religious convictions is wholly intolerable. Even the bare mention of a citizen’s religion in official documents should unquestionably be eliminated. No subsidies should be granted to the established church nor state allowances made to ecclesiastical and religious societies. These should become absolutely free associations of like-minded citizens, associations independent of the state. Only the complete fulfilment of these demands can put an end to the shameful and accursed past when the church lived in feudal dependence on the state, and Russian citizens lived in feudal dependence on the established church, when medieval, inquisitorial laws (to this day remaining in our criminal codes and on our statute-books) were in existence and were applied, persecuting men for their belief or disbelief, violating men’s consciences, and linking cosy government jobs and government-derived incomes with the dispensation of this or that dope by the established church. Complete separation of Church and State is what the socialist proletariat demands of the modern state and the modern church.

The Russian revolution must put this demand into effect as a necessary component of political freedom. In this respect, the Russian revolution is in a particularly favourable position, since the revolting officialism of the police-ridden feudal autocracy has called forth discontent, unrest and indignation even among the clergy. However abject, however ignorant Russian Orthodox clergymen may have been, even they have now been awakened by the thunder of the downfall of the old, medieval order in Russia. Even they are joining in the demand for freedom, are protesting against bureaucratic practices and officialism, against the spying for the police imposed on the “servants of God”. We socialists must lend this movement our support, carrying the demands of honest and sincere members of the clergy to their conclusion, making them stick to their words about freedom, demanding that they should resolutely break all ties between religion and the police. Either you are sincere, in which case you must stand for the complete separation of Church and State and of School and Church, for religion to be declared wholly and absolutely a private affair. Or you do not accept these consistent demands for freedom, in which case you evidently are still held captive by the traditions of the inquisition, in which case you evidently still cling to your cosy government jobs and government-derived incomes, in which case you evidently do not believe in the spiritual power of your weapon and continue to take bribes from the state. And in that case the class-conscious workers of all Russia declare merciless war on you.”

– V.I. Lenin, “The Attitude of the Workers’ Party Towards Religion”