I assume you are referring to Beatty’s lecture when he visits Montag at home. What I find interesting about Beatty in this scene and in the work as a whole is that while he is in charge of the destruction of books, he is also very well read.
I believe Beatty knows and understands what Montag is going through before Montag does. Beatty is essentially intimidating Montag into following the norm. He doesn’t outright accuse Montag of stealing books, but alludes to it.
Beatty’s message in the lecture is that books cause confusion and conflict in society and in man’s personal life. Books offer no answers only questions; therefore, they are dangerous.