So what do you propose to rename Judaism and Hellenism to?
Slavic, or Slavic paganism, is what it is called. Yes, it's what we call it nowadays, not what they called themselves back then, but so is Byzantine Empire, for example. Which even has a contemporary name of Roman Empire, but they still use the modern name to distinguish from the Roman Empire before the fall of Rome. Slavic paganism, unlike Byzantine Empire, doesn't even have a contemporary name. The only name it has is the modern one - "Slavic paganism". It is the only name it has and there it just no other. So that's what should be used then. Or, as second best, something similar like Slavism.
Besides, now that it's discovered that the religion in the game is called Slavic, only the faith is poganstvo, perhaps the developers didn't exactly aim to distinguish the faith from culture, but from the name of the religion? Though it brings the question of how they solved it for other faiths that are presumably the only ones in their religion, like Hellenism and Kushite (which also happen to be named after cultures, demonstrating it's okay to do that).
P.S. Volkhovstvo does sound better than volkhstvo now that it's brought up.