In Chapterhouse Dune, one of the encyclopedia exposita entries at the beginning of a chapter is (boldface mine):
Our household god is this thing we carry forward generation after generation: our message for humankind if it matures. The closest thing we have to a household goddess is a failed Reverend Mother—Chenoeh there in her niche.
While I am aware that maturing humanity is a central goal of the Bene Gesserit, I cannot remember any other mention of the Bene Gesserit’s message for humanity in Frank Herbert’s books. I might have missed something though and I’ve not read anything not written by Frank Herbert. Thus my question is: Is this message ever mentioned again or are its contents elaborated?