The Tom Robinson case is at the core of the ethical convictions of Atticus Finch, as well as the oath that he took when he became a lawyer.

At the end of Chapter 20 as Atticus Finch makes his closing remarks at the trial of Tom Robinson, he reminds the jury of the significance of the judicial system in America: "In this country our courts are the great levelers, and in our courts, all men are created equal."

In Chapter 9 it becomes apparent that Atticus realizes he is bound to seek justice for all. Having been assigned to the Tom Robinson case, Atticus is ethically obligated to seek justice for Tom despite the condemnation of this man in "the secret courts of men's hearts" where racial bias lies. Atticus knows that his family will be subjected to insults and condemnation, but his conscience and his lawyer's oath dictate that he defend Tom. It is because of his obligations as a lawyer that Atticus remarks to his brother Jack that he could not face his children if he refused Judge Taylor and did not take Tom's case. Clearly, all the ethical lessons he has given Jem and Scout would be contradicted by such an action. It is because of the obligatory nature of his position as defense attorney for Tom Robinson that Atticus later tells Scout that this case "goes to the essence of a man's conscience" (Ch. 11).

This question gets to the heart of one of the main themes of the book. The case of Tom Robinson affects Atticus in the sense that it tests his convictions and character. 

Right from the beginning Atticus knows that the case will be a difficult one. He knows that he will probably lose and that his family will suffer a certain amount of hardship on account of his defense of Tom Robinson. Here is what he says in a conversation with Scout:

“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.

From these words, it is clear that there is nothing easy about defending Tom Robinson. The easier thing for him to do would be to let the case pass by. However, if he did that, he would be going against his beliefs. He would allow an innocent man to be punished by a racist jury. Moreover, he would not be demonstrating courage, when he calls his children to be courageous. This inconsistency would damage his conscience. So, he defends Tom Robinson. And by doing so, he remains true to himself and his convictions. He becomes even a better man.