Jem may be a young boy, but he is mature and intelligent enough to recognize that Tom Robinson is an innocent man, and that the jury deliberately disregarded the evidence presented and convicted him anyway. Jem no longer trusts the good people of Maycomb, who he considered
"... the best folks in the world, least that's what they seemed like." (Chapter 22)
He wonders if Tom or Atticus have any real friends or supporters in Maycomb, and his new "fatalistic" view includes the realization that
"... can't any Christian judges an' lawyers make up for heathen juries... Soon's I get grown..." (Chapter 22)
Miss Maudie's positive reinforcement doesn't seem to have any effect on Jem, but it becomes obvious that Jem is looking to the future. When he is grown, he speculates, he will be able to make a difference--either as a lawyer, like Atticus, or as an honest juror who lives up to the court's expectations of making a fair, unbiased decision.