In the Analects, Confucius says of his disciple Zilu: “You has ascended to the hall, but he has not yet entered the inner room.” (由也升堂矣，未入于室也) The idiom 登堂入室, to ascend the hall and enter the room, describes reaching a high level. The Handian dictionary adds that it can also mean receiving a master’s teaching in depth.
An 入室弟子, a disciple of the inner room, is one who has received the master’s true teaching. The dictionary’s example comes from the Book of Jin, about the scholar Yang Ke: “Even students who received his instruction could not speak with him in person unless they were disciples of the inner room.”