Atticus believes in justice and the justice system. He doesn't like criminal law, yet he accepts the appointment to Tom Robinson's case. He knows before he begins that he's going to lose this case, but that doesn't stop him from giving Tom the strongest defense he possibly can. And, importantly, Atticus doesn't put so much effort into Tom's case because he's an African American, but because he is innocent. Atticus feels that the justice system should be color blind, and he defends Tom as an innocent man, not a man of color.
Atticus is the adult character least infected by prejudice in the novel. He has no problem with his children attending Calpurnia's church, or with a black woman essentially raising his children. He admonishes Scout not to use racial slurs, and is careful to always use the terms acceptable for his time and culture. He goes to Helen's home to tell her of Tom's death, which means a white man spending time in the black community. Other men in town would've sent a messenger and left it at that. His lack of prejudice doesn't apply only to other races, however. He is unaffected by Mrs. Dubose's caustic tongue, Miss Stephanie Crawford's catty gossip, and even Walter Cunningham's thinly veiled threat on his life. He doesn't retaliate when Bob Ewell spits in his face because he understands that he has wounded Ewell's pride — the only real possession this man has. Atticus accepts these people because he is an expert at "climb[ing] into [other people's] skin and walk[ing] around in it."
