Religion of the bourgeoisie, ruling class and oppressors will be swept away (not immediately, but with time, as old tendencies typically need to be "unlearned") after their orders are deposed. Religion of the masses belonging to a society with a socialist/communist economic base would look far different than what humankind has experienced hitherto. Reactionary thought and tendencies will be actively combatted across many facets of social life, so there's no doubt that those within religion, too, will be challenged and removed from the differing practices.