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]
(..) Joh Fredersen has been evesdropping... Joh Fredersen breaks into Rotwang's attic. He battles with his old rival, overcoming him. Maria is free. "...faithful after all..." "Come on, Freder! We still have the workers' city." "Women and men, let no one miss today-! Death to the machines-!!" (..)1927 Metropolis
(..) go at once to the workers' city..." "No. , go back to your machine and forget this ever happened - understood?" "I must have someone who is faithful to me, Josaphat, how else can I fulfil my destiny?!" "I must go, Josaphat,- I must look on my own for the person to whom was supposed to lead me." (..)1927 Metropolis