Atticus takes Tom Robinson’s case because he has to and because he believes that it is the right thing to do.
Tom Robinson’s trial throws all of Maycomb into turmoil. A black man, Tom Robinson, is accused of raping a white woman, Mayella Ewell. Mayella is very poor, and Tom of course is black, so they live near each other on the outskirts of town, by the dump.
Atticus is assigned the case by Judge Taylor. He is one of Maycomb’s most respected lawyers. He takes it not just because he is assigned to it, but also because he wants to show his children that racism is wrong.
But do you think I could face my children otherwise? You know what’s going to happen as well as I do, Jack, and I hope and pray I can get Jem and Scout through it without bitterness, and most of all, without catching Maycomb’s usual disease. (Ch. 9)
Atticus also takes the case because he wants to show his children that sometimes you have to fight really hard for something, even if you may not be successful in the end. That is what the trial is for him. It is an important case where he has to work hard to win when he knows that he can’t possibly get an acquittal in the climate of racism.
Atticus uses the example of Mrs. Dubose, who weaned herself off of her morphine addiction, to show why fighting the impossible fight is so important.
I wanted you to see what real courage is, instead of getting the idea that courage is a man with a gun in his hand. It’s when you know you’re licked before you begin but you begin anyway and you see it through no matter what. You rarely win, but sometimes you do. (Ch. 11)
Atticus explains to Scout that defending Tom Robinson is a matter of pride for him. When she asks him why he takes the case, he tells her that he has a lot 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 again.” (Ch. 9)
Atticus feels like he is setting a bad example for his children if he backs down. He is a lawyer, and lawyers often take difficult cases and cases they don't think they can win. He also has to do what is right, for his children’s sake.
Atticus reiterates to Scout that the case is unwinnable, but he has to fight for it anyway.
“Atticus, are we going to win it?”
“No, honey.”
“Then why—”
“Simply because we were licked a hundred years before we started is no reason
for us not to try to win,” Atticus said. (Ch. 9)
Scout doesn’t understand why he would try an unwinnable case, but Atticus explains to her that even when you buck tradition, you still need to do what you believe in. Just because something is tradition does not make it right. Racial prejudice is a perfect example of that.
Another important point is that the innocent deserve to be protected. Atticus introduces this to Scout and Jem metaphorically, when he gives them guns.
Atticus said to Jem one day, “I’d rather you shot at tin cans in the back yard, but I know you’ll go after birds. Shoot all the bluejays you want, if you can hit ‘em, but remember it’s a sin to kill a mockingbird.” (Ch. 10)
Miss Maudie explains that mockingbirds do not hurt anyone. Killing a mockingbird means killing a defenseless creature that brings beauty to people’s lives through its singing and does no harm to anyone. The metaphorical mockingbirds in the story are Tom Robinson and Boo Radley. Each is defenseless, targeted by his society for no other reason than that he is different.
During the trial, Atticus explains to the jury why they should acquit based on guilt or innocence and ignore race in their decision.
Gentlemen, a court is no better than each man of you sitting before me on this jury. A court is only as sound as its jury, and a jury is only as sound as the men who make it up. (Ch. 20)
Atticus tells the court that he believes that they can do the right thing. In a way, they do. They actually deliberate, whereas most black men would be tossed in jail without a thought. Atticus raised reasonable doubt, but not enough to overcome centuries of racism.
These six quotes demonstrate why Atticus Finch had to take Tom Robinson's case, and defend it to the best of his ability. Most other lawyers put in his position would balk at the task or give up because it was impossible. Atticus was not able to get an acquittal, but he was able to get a deliberation. That is a small victory, but it is a victory.