In Atticus's closing remarks, he condemns racial prejudice by denouncing the "evil assumption" that all black men are immoral individuals. Atticus comments that every human in the courtroom, regardless of race, gender, or age, has committed an immoral act. He is fully aware that the jury consists of racist men, who view Tom Robinson as a danger to their society simply because he is black. These racist men abide by the "evil assumption" and have already made up their minds that Tom Robinson is guilty of the crime. During Atticus's closing remarks, he challenges their racist ideology by arguing that every person is inherently flawed, and he urges them to judge the case fairly. Atticus then quotes the Declaration of Independence by saying that all men are created equal and deserve the same treatment in a court of law. He proceeds to encourage the jurors to look past their prejudiced beliefs and judge the case fairly without racial bias. Atticus's final plea is for the jury to do their duty by reviewing the case "without passion" and, in doing so, restore Tom Robinson back to his family.

Atticus, at the end of Chapter Twenty when he addresses the jury in his closing remarks, actually very cleverly condemns the jury themselves and the decision he strongly suspects that he know they will make. Even though Atticus has done a compelling and a convincing job of proving Tom Robinson to be innocent, he strongly suspects that the jury will decide against Tom. Note how he bases his final remarks around the remarks of Thomas Jefferson concerning his words about all being equal:

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.

Atticus' confidence in the jury shows that he is trying to encourage them to do what they know what is right and just rather than what they feel they must do because of the dictates of society. By refusing to do so, this speech actually acts as a condemnation of the jury and their decision, because the jury have shown themselves to be "unsound," in Atticus' terms, and therefore threatening the justice system and its equality that Jefferson spoke so passionately about.

Atticus is condemning racism in his closing remarks, and he asks the jury to do their duty and acquit Tom Robinson.

Atticus is telling the jury that they know Tom Robinson is innocent, and they should not convict him just because he is black.  He is reminding them that racism pervades Maycomb and society, but they do not have to give into it.  In short, he is not condemning them- he is condemning racism.

After laying out a case that Tom Robinson could not possibly have committed the crime he was accused of, raping white girl Mayella Ewell, Atticus using his closing remarks to remind the jury that they are part of a legal process.

Atticus’s message to the jury is that they can’t allow racism to interfere with the profound duty they have to be fair.  He reminds them what an important institution the courts are, and the importance of a fair justice system.

"I'm no idealist to believe firmly in the integrity of our courts and in the jury system- that is no ideal to me, it is a living, working reality. Gentlemen, a court is no better than each man of you sitting before me on this jury….” (Ch. 20)

A big part of Atticus’s speech is to remind the men on the jury that there are good black men and good white men.  Just because a man is black does not make him a criminal.  Just because a white woman accuses a black man of a crime does not mean he should be convicted of it.  Atticus knows, and the jury knows, that convictions usually come automatically in these cases.

Realizing that he is up against impossible odds, Atticus does all he can to remind the men that they do have a choice.  He leaves them with the parting plea: “In the name of God, do your duty."  Then he whispers under his breath, begging the jury to believe him.

Atticus is actually condemning racism and ignorance in his closing argument at Tom’s trial. He begins by reminding the jury that the case is “as simple as black and white.” He follows up by pointing out that there is no medical evidence to prove that Tom committed a crime, but that someone else in the courtroom is guilty. Atticus states that Mayella accused Tom because she felt guilty for breaking society’s code: she is white and she kissed Tom, who is black. Then, he says, she accused Tom of raping her because she needed to “destroy the evidence of her offence.” Atticus suggests that Bob beat his daughter for having broken that code because that’s what he thought any respectable white man should do. Thus, Tom has had to stand up against two white people’s accusations knowing that he has no chance against the lower-class Ewells.

Atticus further argues that the Ewells expected society to support their “evil assumption” that all black people are immoral and untrustworthy. He points out that there is no one in the room who has not lied or committed some immoral act, regardless of race. His point is that society’s code reveals an ignorance that must be acknowledged in order to be corrected. Everyone is part of one race, “the human race.”

Atticus ends his speech by asking his audience to remember that while “all men are created equal,” they’re really not equal. Some people are smarter or have better opportunities in life, which takes away true equality in society. However, the one place that everyone is truly equal is in court. Atticus begs the jury to judge fairly in the courtroom because he believes in the justice system’s ability to protect the innocent. He reminds the jury members to leave their personal opinions out of their decision:

In the name of God, do your duty.

In a bold move, Atticus challenges the prevalent racial prejudice that disfigures public life in Maycomb, and which he knows full well will convict his client of a crime he didn't commit. Atticus can't change the minds of the jury; he knows that. But what he can at least do is get to them to think about the nature of their instinctive racial prejudice, something they've almost certainly never done before.

And so he challenges the members of the jury to confront the evil assumption that a black man accused for raping a white woman is automatically guilty, irrespective of the evidence. He does this by making an eloquent plea for justice based on The Declaration of Independence, which famously states that all men are created equal.

That being the case, Tom Robinson is as deserving of justice as anyone. But this will only be delivered if—and it's a very big if—the members of the jury do their duty and decide the case without "passion", which, in the present context, means prejudice.

Of course, Atticus is no fool. He knows full well that the jury has already made its decision to convict. But he's determined to give Tom the very best legal defense that he can give, and by the time he's finished giving his inspiring closing speech to the jury, no one can be in any doubt that that's precisely what he's done.

Atticus is condeming society, racism, and the law.  There would not even be a trial if Tom Robinson were white.  There is not any evidence in the trial to convict Tom, but the town and Bob Ewell would convict Mayella of lusting after an African-American male.  Atticus reminds the court that there is one place where "all men are created equal" and that is under the law.  He begs the jury to do their duty, "in the name of God," which evokes their religious sensibilities.  In order to do the right thing, the jury must suspend their individual beliefs to save the life of one innocent man caught in a world that shuns his existence.