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.