Jem's quote is a response to the little talk he has had with Miss Maudie shortly after the trial of Tom Robinson. Jem is highly upset about the jury's verdict, since he believes that Atticus clearly proves that Tom is innocent of the charges against him. Jem feels that both Atticus and Tom must be alone in Maycomb, with no real supporters aside from their families and a few friends. He also questions the validity of "heathen juries," but Maudie points out that Atticus and Tom have many friends in the town. She explains that aside from Tom's "colored friends," Judge Taylor deliberately selected Atticus in order to give Tom the best representation possible. She also explains that the verdict is a "baby-step" in local racial relations. Atticus is
"... the only man in these parts who can keep a jury out so long in a case like that... we're making a step--it's just a baby-step, but it's a step." (Chapter 22)
But Maudie's hopeful attitude doesn't soothe Jem's anger, and his response--about Christian judges and lawyers like John Taylor and Atticus not making up for the verdicts of "heathen juries"--is one of frustration. But it also reveals that Jem, who wants to follow in Atticus's footsteps as a lawyer, is thinking to the future when he can make a difference.
"Soon's I get grown--" (Chapter 22)