Yeah good points. Only answer I can think of is that the common people were not educated back then. And the satanist killed anyone preaching the truth. So they had to go under ground to keep the truth alive at all.
The weakness of that is if the evil ones find out about your groups they can infiltrate and turn them to working in the opposite.
But obviously the time for any secret groups has come to an end. We can not afford the corruption. The stove piping. The isolationg people on a need to know basis allows people to work for groups they think are helping their country and they are actually working for evil itself. Satanic level evil.