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]
(..) to do with the robbery? Didn't you? You little beast. Three - No. Four years ago I swore to you that I was retired. I've been faithful to you ever since. In your fashion. What the hell does that mean? That you're being teased. I believe you, darling. (..)1975 The Return of the Pink Panther