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]
(..) was save us some trouble. Both: Now it's your turn. (grunts) ( screams ) I was-- I was faithful to you. Why couldn't you be? Rick! ( Footsteps approaching ) ( gasps ) ( brief gasp ) (..)2009 The Black Waters of Echo's Pond