I've had a little exposure to secret societies.
The fraternal organizations, like the Moose Club. They sponsored my Boy Scout troop. Dad would come and pick me up after meetings and have a beer at the bar while he waited for me. After a while, they said, "Hey, join our club." He looked at their literature, found out they were proud of being an all-white club, and snarkily told them he didn't quality because we have Native American in the family lines.
Speaking of Boy Scouts, I was part of the Order of the Arrow, a kinda secret subset of the Scouts. It didn't last long, because I drifted from the Scouts soon after.
I belonged to the Elks for a while for no other reason than they had a nice workout room that I used during lunches. I had to go through this extremely weird ritual to be an Elk. Once the Elks sold their building and I found a different fitness club downtown, I was no longer an Elk.
I've been invited to meetings of the Masons and Knights of Columbus. Grandma, in her youth, was invited to some Star thing which was a young ladies' Mason group. We thought the Masons and Stars were way too serious and arrogant, and I couldn't stand the Knights' outfits.
When I was growing up, my buds and I would form "secret" clubs. I dunno. It seems like the stuff I mentioned up above is an extension of that little-boy thing. I hope I'm not offending anyone. When you don't take it real seriously, it seems kinda silly.
And Dan Brown is a guy who comes up with stellar storylines and mucks it up with hyperbolic hack writing.