Why does Jem think that Atticus will win the case?

Jem naively believes that Tom Robinson will be acquitted after the jury listens to the testimonies and discusses the evidence presented during the case. As an innocent child who is unaware of the prevalent prejudice throughout Maycomb's community, Jem naively believes that the jurors will rule in favor of Tom Robinson. Atticus has presented a competent, clear case arguing that Bob Ewell is responsible for his daughter's wounds and defending Tom's innocence by accurately portraying Mayella as a guilt-ridden young woman. Given the lack of medical evidence, Tom's handicap, and Bob Ewell's motivation to physically punish his daughter, Jem believes that there is no way the jury will convict Tom. In chapter 20, Scout enters the courtroom before Atticus's closing arguments, and Jem tells her:

He’s just gone over the evidence . . . and we’re gonna win, Scout. I don’t see how we can’t. He’s been at it ‘bout five minutes. He made it as plain and easy as—well, as I’da explained it to you. You could’ve understood it, even (Lee, 206).

Unfortunately, Jem does not take into consideration the jury's prejudiced beliefs and the racist ideology of Maycomb's community. When Judge Taylor reads the verdict, Jem is stunned and disappointed. He immediately begins to cry and laments that the court's ruling was unfair.