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]
(..) Even the children know it. Your daughter... Why you won't just name the father... You've brought sin amongst us. The faithful must gather in judgment... ...for we are called to purify this filth. What does she mean, Mommy? You're weak, my dear sister. You always were. (..)2006 Silent Hill