The quote you want is in the second book, "The Sieve and the Sand", when Montag visits Faber at his home and asks him to help him understand what he reads. Beatty has confused him by telling him that books will create chaos in the society, and Montag wants to know more.
My edition of the book has the quote on page 82 at the very top of the page. It is the first line. Your edition of the book may have it on another page but it should be close. The paragraph starts with
"It's been a long time, I'm not a religious man." (pg 81)
Faber thinks of himself as a coward because he did nothing and said nothing when he saw the way things were going. He says,
"Mr. Montag, you are looking at a coward. I saw the way things were going, a long time back I said nothing. I'm one of the innocents who could have spoken up and out when no one would listen to the 'guilty, but I did not speak and thus became guilty myself." (pg 82)
On page 86 of Simon & Schuster's 60th Anniversary Edition of Ray Bradbury's Fahrenheit 451, Faber tells Montag, "I am a cowardly old fool." In Part Two of the novel, Montag travels to Faber's home to ask him for help understanding various texts. When Montag arrives, Faber explains to him that he is not interested in helping him challenge the authoritative government. Faber fears for his safety and well-being which is why he has chosen to remain silent over the years. After Montag guilts Faber into elaborating on the contents and importance of books, Faber decides to show Montag a secret room in his home where he has made a two-way communication device called the "green bullet." As Faber leads Montag into the secret chamber, he tells Montag that the green bullet is proof of his cowardice. For years, Faber has contemplated the idea of revolution and even produced a communication device that would aid in the revolutionary cause.