Atticus tells the jury that in a court of law, "all men are created equal." A court is, however, no better than the members of its jury, and he urges the jury to do their duty.
In Chapter 20, Atticus appeals to the jury's sense of dignity, and in putting together the facts of the case, he stresses the simplicity of the evidence and shows that the facts point toward Tom's innocence. As later becomes apparent, Atticus doesn't really believe that the jury will set Tom free, even though he hopes they will, as evidenced by his final statement, under his breath, "In the name of God, believe him." All Atticus can hope for is to leave an impression upon the town by exposing the truth for all to see.