Holy of Holies

noun3

1. •
   The most sacred place within a sacred building.
2. •
   (figurative) That which is holiest among holy things.

   • Rabbi Akiba declared that if the other books of the Bible are holy, Shir Hashirim is the "holy of holies." — 1996
3. •
   (informal, humorous) One's private retreat, an inner sanctum.