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]
(..) He came to us from King's Landing. A maester of the Citadel, chained and sworn, and sworn brother of the Night's Watch, ever faithful. At the Wall, a dozen lord commanders came and went during his years of service, but he was always there (..)2015 TvShow: Game of Thrones Title: The Gift Season: 5 Episode 7