(Book 2 Chapters 7-13) Why does Dickens call the monseigneur’s private rooms the Holiest of the Holiests? What is the sarcasm in this?

The corruption of the upper class is conveyed through the ironic religious language used to describe Monseigneur. His personal room is called the "Holiest of Holies," a direct translation from the Latin sanctum sanctorum and ultimately from the Hebrew Bible, referring to the especially sacred inner chamber of a temple.