Wikipedia page on this says it best:

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.

https://en.wikipedia.org/wiki/Marxism_and_religion