The phrase "great secret" refers to a piece of hidden knowledge that is extremely important or valuable, but known by only a few people. Full definition
Good marketing is one great secret of successful self publishers. (readersfavorite.com)
It may have the look (loosely) but carries with it none of the heart, soul or charm of the world's greatest secret agent. (metacritic.com)
But you're still skeptical, just because everyone else thinks they're the cat's PJs isn't proof that they're not just the latest fad in a «if it doesn't work immediately on to the next great secret weapon» world. (greensmoothieforbeauty.com)