When Beatty comes to visit Montag, he does so because he imagines something is wrong with Montag. He anticipated Montag would be "sick." When Beatty arrives, he begins to tell Montag about the history of firemen. He tells Montag that he understands and that every once in a while, a fireman will get curious and take a book. He also lets Montag know that a book returned in 24 hours is done so with no questions asked. He shares the 5 rules of firemen. Of most importance, Beatty tells Montag about how the world has come to work correctly as a result of the firemen as this truth plays out:
Not everyone is born free and equal, as the Constitution says, but everyone is made equal.
This passage enables us to understand the dystopian nature of the society, and we react with shock as Montag does. Montag tries to imagine how he can get out of his job for the rest of his life. His gut instinct is to run because he feels something physically wrong with what they do.