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]
(..) old Mary Jane Rottencrotch... ...through her purty pink panties are over. You're married to this piece. This weapon of iron and wood. And you will be faithful. Arms. Prepare to mount. Mount. Port. Pray. (..)1987 Full Metal Jacket