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.