In Novaya Zhizn No. 28, 3 December 1905, Marxists Internet Archive, Lenin said that: “Religion is one of the forms of spiritual oppression, which everywhere weighs down heavily upon the masses of the people, over-burdened by their perpetual work for others, by want and isolation . . . Those who toil and live in want all their lives are taught, by religion, to be submissive and patient while here on Earth, and to take comfort in the hope of a heavenly reward. . . . Religion is opium for the people. Religion is a sort of spiritual booze, in which the slaves of capital drown their human image, their demand for a life more or less worthy of Man.” Marxists Internet Archive
Feuerbach, Ludwig (1841). "Chapter XVI. The Distinction between Christianity and Heathenism". The Essence of Christianity. Translated by Eliot, George – via marxists.org.
Marx, Karl (1932) [1845–6]. "Part I: Feuerbach: Opposition of the Materialist and Idealist Outlook. A: Idealism and Materialism". The German Ideology. Marx-Engels Institute, Moscow – via marxists.org.
Lenin, V.I. (1965) [1905]. "Socialism and Religion". Lenin Collected Works. Vol. 10. Moscow: Progress Publishers. pp. 83–87 – via www.marxists.org.
Lenin, Vladimir Ilyich (1972) [1922]. "On the Significance of Militant Materialism". Lenin's Collected Works. Vol. 33. Translated by David Skvirsky; George Hanna. Moscow: Progress Publishers. pp. 227–236 – via marxists.org.