19th-century German philosopher Karl Marx, the founder and primary theorist of Marxism, viewed religion as "the soul of soulless conditions" or the "opium of the people". He believed religion survives because of oppressive social conditions. When this oppressive and exploitative condition is destroyed, religion will become unnecessary, according to Marx. At the same time, he saw religion as a form of working-class protest against poor economic conditions and alienation. Denys Turner, a scholar of Marx and historical theology, classified Marx's views as adhering to post-theism, a philosophical position that regards worshiping deities as an eventually obsolete, but temporarily necessary, stage in humanity's historical spiritual development.

In his interpretation and synthesis of Marx, Vladimir Lenin theorized state-sanctioned religion as an ideological apparatus of the bourgeoisie, and its places of worship as institutions providing justification for ruling ideas to the working class, while retaining Marx's view of religion's dual role as proletarian protest and solace. A number of Marxist-Leninist governments in the 20th century, such as the Soviet Union after Vladimir Lenin and the People's Republic of China headed by Mao Zedong, implemented state atheism to allow the "opium" to be decentralized in secular civics, in line with post-theism.

Karl Marx's religious views have been the subject of much interpretation. In the Critique of Hegel's Philosophy of Right of 1843, Marx said religion is a man-made concept and, as such, reflects human conditions. He saw religion as an "inverted consciousness of the world" reflecting an inverted, unjust society. He argued that religion is both an expression of distress and a protest against the real distress. "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," Marx wrote. The abolition of religion as "illusory happiness" would lead to real happiness, he said — but only if people were freed from oppressive material conditions.

Criticism has plucked the imaginary flowers on the chain not in order that man shall continue to bear that chain without fantasy or consolation, but so that he shall throw off the chain and pluck the living flower.

— Karl Marx, A Contribution to the Critique of Hegel’s Philosophy of Right

According to Howard Zinn, "[t]his helps us understand the mass appeal of the religious charlatans of the television screen, as well as the work of Liberation Theology in joining the soulfulness of religion to the energy of revolutionary movements in miserably poor countries". Some recent scholarship has suggested that "opium of the people" is itself a dialectical metaphor, a "protest" and an "expression" of suffering.

Some view the early Christian Church, such as the one described in the Acts of the Apostles, as an early form of communism and religious socialism. They believe communism was just Christianity in practice and Jesus was the first communist. This link was highlighted in one of Marx's early writings, in which he wrote: "As Christ is the intermediary unto whom man unburdens all his divinity, all his religious bonds, so the state is the mediator unto which he transfers all his Godlessness, all his human liberty". Furthermore, Thomas Müntzer led a large Anabaptist communist movement during the German Peasants' War which Friedrich Engels analysed in The Peasant War in Germany. The Marxist ethos that aims for unity reflects the Christian universalist teaching that humankind is one and that there is only one God who does not discriminate among people. Tristram Hunt attributes a religious persuasion to Engels.

Roland Boer asserts that Marx's depiction of religion as 'opium', while suspicious of religion's addictive potential, also emphasizes religion's medicinal properties akin to those of opium in Western medicine.