Religion is a sort of spiritual booze...

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.

(Wladimir Iljitsch Uljanow in ‘Socialism and Religion’ from 1905)

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. (Contribution to the Critique of Hegel’s Philosophy of Law: Introduction. In Marx and Engels Collected Works, pp. 175-6)