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]
(..) enemies abroad and at home. I fear for his safe return. Fear not, dear Sheriff. He will return. And when he does, he will wish to reward his faithful subjects. I asked your father three times if he worshipped the dark one. Because his answer vexed me so grievously. He said... (..)1991 Robin Hood: Prince of Thieves
(..) 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
(..) ...that, in Casablanca, human life is cheap. Good night, mademoiselle. - What happened with Rick? - We'll discuss it later. Our faithful friend is still there. Please don't go to the underground meeting. I must. It isn't often that a man gets to display heroics before his wife. (..)1942 Casablanca