How do you keep a giant secret for 2000+ years?

I know this question will seem very vague as I try to phrase what I mean, but I’ll do my best.

Has anyone ever read stories or watched movies where people who have deicated their lives to keeping some ancient secret manage to do so for hundreds, even thousands of years? Secret dragon protectors, secret orders who guard an artifact, government/political secrets, etc. I wish I knew of some good examples, but I can’t recall any at the moment…

But if you know what I’m talking about…do you know of any good stories that are based on a big secret(s) or conspiracy, particularly in the fantasy and science fiction genres? If so, what made keeping this big secret for so long believable for you? What made it work, in your eyes?