Why does Atticus agree to take the Tom Robinson case? Which reason seems the most important to you?

Set during the Great Depression and written during the Civil Rights Movement, To Kill a Mockingbird attacks the racial injustices inherent in the Southern courts. Tom Robinson's case is similar to that of the "Scottsboro Boys," nine young African-Americans falsely accused of raping two white women. 

In taking Robinson's case, Atticus does what should be done, something no one else had the moral fortitude to do. For this, he and his household face harsh criticism from the townspeople and even their own family. 

In a cowardly fashion, the attacks are often aimed at his children rather than at Atticus himself. Elderly neighbor Mrs. Dubose tells Jem that Atticus laws "for niggers and trash." In addition, Scout faces a classmate and her cousin, who, in separate incidents, call her father a "nigger-lover." 

When Scout questions Atticus, he asserts, "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." He adds that he could not ask his children to obey him: 

“Because I could never ask you to mind me again. Scout, simply by the nature of the work, every lawyer gets at least one case in his lifetime that affects him personally. This one’s mine, I guess." 

Later, when challenged by his brother, Atticus says, 

“Right. 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."

Atticus knows if he does not argue for what is right in the courtroom, he has no right asking his children to do right in his home and in public. As their father and foremost role-model, he must embody that which he asks his children to do. 

In the courtroom, he argues for the truth, emphasizing Tom's innocence. He cannot stand by and watch a man be unjustly convicted simply on the basis of skin color. 

“I have nothing but pity in my heart for the chief witness for the state, but my pity does not extend so far as to her putting a man’s life at stake, which she has done in an effort to get rid of her own guilt."