Secret Society
The name would indicate a society, or group of individuals, that keep their existence secret from the masses, or at least from the established religions or governments.
If a secret society does a good job of being 'secret', then the rest of the world would not even know that they existed. One the existence of a secret society becomes known, they cease to be secret.
Many people have accused the Freemasons as being a secret society. Is Freemasonry a Secret Society?
If the Freemasons are attempting to be a secret society, they have totally failed in that goal.