When Scout first hears about her father taking Tom Robinson as a client to defend in court, it's not from Atticus. Cecil Jacobs tells her at school in chapter 9, and he doesn't say it to compliment her. Scout is confused and asks her father what Cecil was talking about. Atticus tells her that some people in Maycomb don't believe that he should be defending a black man. Scout asks why he is doing it, then. Atticus says the following:
"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" (75).
This passage shows Atticus's deep commitment to the case because he knows that it represents the problems that both Maycomb and the whole South face--discrimination and prejudice based on race. Atticus could simply show up to court and not do much by way of a good defense for Tom, and no one would think any less of him; but that's not who he is. Atticus wants to give Tom the best defense because otherwise, he wouldn't have one at all. Even people who are on his side think Atticus has a lot to lose from defending Tom. Link Deas even says so in chapter 15. To that Atticus says the following with determination:
"Link, that boy might go to the chair, but he's not going till the truth's told . . . And you know what the truth is" (146).
This quote shows that Atticus won't let anything get in the way of Tom having his day in court. He is determined to tell the truth that Tom did not rape Mayella Ewell no matter if the traditions and prejudices of the county are facing him. In fact, Atticus makes it known during his final speech that a court shouldn't be swayed by prejudice or tradition; rather, a man should be convicted only if the facts prove that he committed a crime. Atticus elaborates as follows:
"But there is one way in this country in which all men are created equal--there is one human institution that makes a pauper the equal of a Rockefeller, the stupid man the equal of an Einstein, and the ignorant man the equal of any college president. That institution, gentlemen, is a court. . . Our courts have their faults, as does any human institution, but in this country our courts are the great levelers, and in our courts all men are created equal" (205).
Because of Atticus's belief in the judicial system, and that courtrooms are where people should receive equal treatment, this is why he is determined to defend Tom Robinson. Each day, black people like Tom go out and face racism, prejudice, and discrimination; but, they shouldn't have to face that in a court of law. The law can't make people treat others with respect, but when it comes to a case to be decided in court, that is where everyone should receive a fair shake. Again, this belief is why Atticus feels he can't turn his back on Tom's case.