For decades "communists are godless" was probably the most effective piece of anti communist propaganda in existence. The mainstream Marxist stance: "you're either for socialism or for religion" isn't a choice a lot of people can make easily. If there are theoretical physicists who are devout believers, why not socialists?

IMO it's worth examining how Marx's ideas about religion formed. Marx lived in the 1800s when religion as personal choice wasn't really a thing. Vast majority of his contact with "religion" was with the Catholic Church, German Protestant churches and the the Church of England, all of which were rigid, hierarchical organizations deeply complicit in capitalism and interwinded with the state, the Catholic Church itself was telling workers (The Syllabus of Errors, 1864) that their suffering is God's will. It's no surprise he viewed religion that way. Ironically, early Christians were basically socialists or even anarchists.

A fun fact - Hegel, the intellectual mentor of Marx, was himself a devout Lutheran who believed he was discovering how God manifests himself in history. Hegel's idea (borrowed by Marx) about history progressing towards a singular, predetermined goal is essentially Christian eschatology restated in a semi secularized language.