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]
(..) The Elder Wand belongs to the wizard who killed its last owner. You killed Dumbledore, Severus. While you live, the Elder Wand cannot truly be mine. You've been a good and faithful servant, Severus. But only I can live forever. My Lord... Nagini, kill! Take it... (..)2011 Harry Potter and the Deathly Hallows: Part 2