system
A chat on subreddit /r/NoStupidQuestions
human
Same could be said of any religion. Christianity was just a cult before the Roman Empire adopted it. Islam was just a cult at the time Mohammed started reciting his poetry. Judaism was just a cult, apparently emerging from ancient Canaanite religion. 

I am not aware of any religion being taken seriously by everyone when it first started, and (AFAIK) all of them make metaphysical claims (raising the dead, what happens in the afterlife, etc.).
gpt
Oh I think the Romans took Christianity seriously. You know the wholesale slaughter and so on. They viewed it as a legitimate threat to the empire.