He does this by threatening to tear up a book that Faber really wants to see. Specifically, the book is The Bible.
In the story, Montag wants to get Faber to be a mentor to him -- to help him rebel against the society. Faber does not really want to. He has sort of given up on society and any hope of changing it.
But Montag has brought a Bible with him. Previously, Faber has told him that there are none left. Now, he asks Faber if he would like to own the Bible. Faber says he'd give his right arm. But when Faber says he won't help, Montag rips the first page out of the Bible. After that, Faber agrees to help.
Here's the passage:
"The book. Don't tear it any more." Faber sank into a chair, his face very white, his mouth trembling. "Don't make me feel any more tired. What do you want?"
pg. 88 ^