Epic Fantasy - Magic users, Gnosys, or Gnosis, maybe Map or Heaven in title?

[Dave]

I'm desperately trying to remember the title for a fairly long book I read in the 1990s or 2000s. One of the central plots centered on one of the few remaining magic users in this world, who had a powerful magic called "gnosis" or "gnosys" or something like that, and his opponents were trying to discover the secret of how his magic worked, because it was much more powerful than theirs. They tried to copy what he said, symbols, gestures, etc., but they couldn't replicate it. I don't want to spoil the ending, but you eventually learn that the secret to his magic was that he was doing something very simple that no one else could see or hear. I feel like Map or Heaven or Sky is in the title, but I could be totally wrong. Any help would be most appreciated. Thanks!