I should point out that Marx certainly wasn't against religion in the same way as his contemporaries like Bruno Bauer or Max Stirner. Those two in particular were edgy fedora style atheists of their time. They saw religion as simple ignorance or society wide brainwashing. They viewed religion as holding tendrils around a person's mind, and once that was broken, a person could achieve freedom. Marx saw a person's freedom as intrinsic to their material and social conditions. Religion is there as medicine to soothe suffering caused by a lack of freedom.

Socialist states have typically had to grapple not simply with religion, but religious organizations. The Orthodox church in the Soviet Union for instance was previously an aspect of the administrative state for Tsarist Russia. Those kind of organs don't simply disappear overnight, they have to be uprooted. The church acted as social infrastructure for Russian/etc peasants. The school, tax collector, bureaucracy, library, hospital. Sometimes all of that was contained within the local church. You can't have a socialist administrative state if you have a remnant of the previous society integrated within people's lives like that. A person's religion should be entirely their own choice, not something they're forced to rely upon.

Cuba as another example bans private religious schools. A democratic society wouldn't have schools that are unaccountable to the public. It's not the responsibility of religious organizations to plug in the gaps. There shouldn't be any need to resort to religion to house the homeless or feed people. China took a huge step and has its own internal branch of Catholicism so there's not a constant flow of money out of their country and into the Vatican. No socialist country has banned religion outright, because there's no need. There is however a need to clamp down on unaccountable organizations acting as reserve social services.