What was Marx criticizing when he said that religion is "the opiate of the masses"?

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Marx believed that religion was used as a tool of the ruling class to control and oppress the working class. He argued that religion distracted the working class from their oppression and exploitation by promising them rewards in the afterlife, while ignoring their suffering in the present. Calling religion the "opiate of the masses" was a way for Marx to criticize this oppressive use of religion.

What Was Marx Criticizing When He Said That Religion Is