My impression is that Christianity was slowly crushed (though not outright destroyed) in the first few decades, but during WW2 restrictions were eased for the parts of the Orthodox Church willing to collaborate, though it was still hardly a religiously free society.
Was this mirrored for Islam?
There's a lot to be said on the subject, but in the meantime you might be interested in this answer I wrote about the Soviet government's relationship to and control of Islam.