But I've noticed a universal and basic misunderstanding of the word "mystery". That is, everyone seems to treat it as if it means "unknown", or even "unknowable". The real meaning is more like "secret", or "that which few know".

You're absolutely right, secret societies and ritual devil worship spawned over there, it all goes back to Enlil and Enki aka Good vs Evil.

But there's only one true creator God that's watching over us, he destroyed Babylon once and he will destroy lucifer again.