What doesn't Montag want to do? Why must he trust Faber?
When Montag visits Faber's apartment, he makes it clear that there are certain things which he does not want to do:
"I don't want to change sides and just be told what to do. There's no reason to change if I do that."
In other words, Montag wants to be allowed to think for himself and to express those thoughts and opinions, whatever they might be. For Montag, this is the whole point of rebellion: to overcome censorship and to create a society in which people have the right to free expression.
As part of his rebellion, Montag must trust Faber in a number of ways. Firstly, he must trust in Faber's listening device to protect against Beatty's pro-censorship rhetoric. Secondly, he must trust in Faber's plan to overthrow the fireman system. As part of this plan, Faber must enlist the help of a former printer that he knows who may be able to print new books. Finally, he must also trust in Faber's belief that the plan will work and that, together, they can undo the damage caused by censorship.
Montag doesn't want to return to work at the Firehouse. He's afraid that when Beatty questions him, he'll give away the fact that not only has he been reading books, but he is starting think for himself and enjoys reading the books. Faber promises to be with him and talk him through the confrontation with Beatty. Montag must trust that Faber will help him talk to Beatty, and that Faber will let him think for himself rather than just telling him what to do and what to think.