He began to keep a journal and was 'faithful to' it for years. [=he continued to write in his journal for many years]
The movie is 'faithful to' the book. [=the story told in the movie closely matches the story told in the book]
He was not 'faithful to' his word. [=he did not do what he said he would]
(..) I beg you, come to King's Landing, swear fealty to King Joffrey and prevent any strife between the great houses of Lannister and Stark Your faithful sister, Sansa. - They forced me to do it. - Did they? With a knife at your throat? Did they put you in a rack and stretched (..)2017 TvShow: Game of Thrones Title: Beyond the Wall Season: 7 Episode 6