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]
(..) ...that, in Casablanca, human life is cheap. Good night, mademoiselle. - What happened with Rick? - We'll discuss it later. Our faithful friend is still there. Please don't go to the underground meeting. I must. It isn't often that a man gets to display heroics before his wife. (..)1942 Casablanca