What point is Faber making when he states “remember Caesar, thou art mortal” (86)?

Faber is warning Montag to be careful about his plan to save books and rebel against the state. Faber is reminding Montag that he is mortal and can easily be killed. This is an allusion to Shakespeare's Julius Caesar when Caesar’s praetorian guard, whispers to Caesar as the parade moves down the street, ‘Remember, Caesar, thou art mortal.’