In the novel, there are several examples of mob mentality and majority rule. As a lawyer, Atticus knows full well that the American justice system is based on majority rule. His job is to try to convince 12 jurors to believe in Tom's innocence, and if he is able to Tom will be free and Atticus will win the case.
In Maycomb, there is also an acceptance and encouragement of racial and economic prejudice. We see this in its ugliest form when the mob shows up at the town jail to try and lynch Tom.
However, Atticus knows that an individual's sense of right and wrong and how they think and feel is not controlled by a jury, the people of Maycomb, or any outside influence. Atticus is the best example of having the courage to stay true to his own beliefs and listening to his conscience.