The trial of Tom Robinson hangs like an enormous weight over the Finch household in the chapters leading up to the actual court proceedings. Atticus, it is made clear in Chapter 9, is representing Tom, who has been unjustly accused of raping a white woman by the town's most virulently racist example of "white trash," Bob Ewell. Scout is troubled by accusations she has heard from others about her father's role in defending Tom, Atticus being an attorney and respected citizen of Maycomb. How and why Atticus came to be in this position, however, is only incrementally revealed. It is in Chapter 9, that Scout challenges her father for the reason he has taken such a highly-divisive case:
“If you shouldn’t be defendin‘ him, then why are you doin’ it?”
“For a number of reasons,” said Atticus. “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 again.”
This sentiment -- and it is repeated in Chapter 11 when Atticus states, "This case, Tom Robinson’s case, is something that goes to the essence of a man’s conscience—Scout, I couldn’t go to church and worship God if I didn’t try to help that man" -- reveals the depth of Atticus' conscience and commitment to do what he believes is the right thing irrespective of its popularity and the ridicule to which it will inevitably expose his family. It is later in the series of exchanges Atticus has with his family, including Atticus' brother Jack, in Chapter 9, that Atticus refers to his appointment to this case by Judge John Taylor:
“Before I’m through, I intend to jar the jury a bit—I think we’ll have a reasonable chance on appeal, though. I really can’t tell at this stage, Jack. You know, I’d hoped to get through life without a case of this kind, but John Taylor pointed at me and said, ‘You’re It.’”
It is in Chapter 9, therefore, that Atticus reveals that he has taken this unpopular case because he was appointed to it by the presiding judge. Atticus could have, conceivably, turned down the judge's request that he defend Tom Robinson, crippled, desperately poor African American. It is emphasized, however, that he accepted the case because of a moral imperative to display for his children and to others his commitment to do what he believes is right.