Atticus tells the jury that there is no place for racism in a courtroom, and they should treat Tom Robinson like any other defendant and acquit him.
Atticus is an outstanding lawyer. He is chosen to defend Tom Robinson because he is an honest man and a hard worker. He is also one of the few men in the town of Maycomb who is not a racist. He actually plans to defend Tom, which is not a popular idea in Maycomb.
And defend Tom he does! Atticus proves that Tom Robinson could not have physically committed the crime he is charged with, because he is a cripple. He proves that the crime never took place at all, no one ever went for a doctor, and Bob Ewell attacked his own daughter because she was friendly with a black man.
In his summation, Atticus begins by saying conversationally that the “case is as simple as black and white” (ch 20). He then reminds the jury that there has been no proof that the crime took place, and there is proof that Mayella’s father is the only guilty party. By accusing Tom Robinson, Mayella was just an educated young lady who “must destroy the evidence of her offense,” meaning Tom Robinson.
"And so a quiet, respectable, humble Negro who had the unmitigated temerity to 'feel sorry' for a white woman has had to put his word against two white people's. (ch 20)
Atticus tells the jury that the idea that all black men are liars is just a lie that white men tell themselves. There are white men who are liars, and there are black men who are liars. Race has nothing to do with it.
Quoting Thomas Jefferson, Atticus comments that the phrase that “all men are created equal” is only true in a courtroom. The courtroom is the great leveler. He ends with this plea.
I am confident that you gentlemen will review without passion the evidence you have heard, come to a decision, and restore this defendant to his family. In the name of God, do your duty." (ch 20)
Of course, Atticus knows that the jury will never acquit. Social convention is too much against it. However, he has made them stop and think—and that is a victory in and of itself.