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]
(..) And only his judgment concerns me. Shall we throw him through the Moon Door? My lord. My lord, I have always been faithful to House Arryn. To your father, to your mother, and now to you. Do you believe him, Uncle Petyr? Lord Royce has served the Vale well. He's enjoyed a distinguished (..)2016 TvShow: Game of Thrones Title: Book of the Stranger Season: 6 Episode 4