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]
(..) Will you scold me? (music) (music) when lost, Will you find me? (music) (music) but they told me (music) (music) a man should be faithful (music) (music) and walk when not able (music) (music) and fight till the end (music) (music) but I'm only human (music) (music) oh, lord (music) (..)1993 Free Willy