Faber is much older than Montag, and he has been around long enough to really be able to reflect upon the changes that have occurred in society. He believes that he is a coward, because when things first started changing, he did nothing to prevent those changes. Thus, they now live in a society where books are burned, and he commends Montag for his bravery in holding on to a physical copy of a Bible, which Faber hasn't seen in ages.
One of the ways Faber influences Montag is by helping him understand that books themselves contain no special magic. Instead, books of quality help people to really examine life. They offer readers the chance to slow down and think about how their lives are progressing and to reflect upon society and truths. Faber maintains that these same qualities could be found elsewhere if people looked hard enough, yet most of their society has been trained to mentally rush through their days with a sense of mindless conclusions. In losing literature, they have forgotten how to really think. This conversation encourages Montag to continue thinking for himself, defying the norms of their society, in his own quest.
Later, Faber quite literally becomes the voice in Montag's ear as they endeavor to break the chains of oppression in their society, offering wisdom and instruction as the two collaborate to construct a better world together.