In chapter 9, Cecil Jacobs tells Scout at school that her father "defends ni***rs". When Scout asks her father about what Cecil meant by that, he says that some people around town don't believe that he should defend Tom Robinson. Scout wonders why he is doing it if people say he shouldn't. Atticus responds with the following:
"For a number of reasons. . . The main one is, if I didn't I couldn't hold up my head in town, I couldn't represent this county in the legislature, I couldn't even tell you or Jem not to do something. . . because I could never ask you to mind me again" (75-76).
Basically, Atticus is saying that he would be a hypocrite if he didn't take Tom Robinson's case. Atticus would be going against his conscience if he didn't stand up for what he knows is right. He knows that a black man deserves a fair trial just like a white man does. Some people in Maycomb don't believe this because they are prejudiced and racist. Atticus knows how people in Maycomb will go to church and preach "love one another," but then treat black people terribly. Atticus won't be like those people because he knows better and he believes better. Thus, if Atticus didn't take the case, then he would be a hypocrite. According to that line of thinking, Atticus also believes that his children wouldn't have to obey a hypocrite; therefore, they wouldn't have to mind him if he turned into one. He would lose credibility as a father if he were a hypocrite, too. It's tough for children to hear dad teach them to stand up for what is right at home, but then he doesn't go and do the same thing himself.