During the trial of Tom Robinson, the townspeople have turned against Atticus because he has taken the case. Atticus has done his job, and proved that Tom is innocent, but because he is a black man, Atticus knows that he will never be treated fairly.
When the closing arguments come about, Atticus tells the people on the jury that they have to look past Tom being a black man and see him as just a man. He tells them it is their God given duty to do the right thing. He is trying to get them to do what is right and not what the town says. Atticus knows it is not going to end well for Tom, so he tries to appeal to the jury's moral compass. By telling the jury that it is their God given right to do the right thing, he is telling them that they have a responsibility to God to do what is right.
"She was white, and tempted a Negro. She did something that in our society is unspeakable: she kissed a black man. Not an old Uncle, but a strong young Negro man. No code mandated to her before she broke it, but it came crashing down on her after words."
With this statement, Atticus is showing that Tom was tempted but wasn't the one who broke the code. He is saying that he knows Tom is a black man, but the jury has to do what's right.