At no point does Luke believe Jabba can defeat him. The journey away from home often consists of the hero saving people from a tyrant or monster, and this is the exact mission Luke has undertaken. Successful completion of this quest will prove Luke's status, because before the completion of this quest, it is obvious that Luke has not yet achieved greatness. Most people do not believe him a true hero. When Chewbacca suggest to Han Solo that Luke is going to save them, Solo's response is, "Luke? Luke's crazy. He can't even take care of himself, much less rescue anybody." Obviously, Luke is not viewed as a hero at this point. However, by completing the perilous journey of rescuing Han Solo, Luke succeeds in gaining respect and proving his status as a Jedi Knight, even from the eyes of his most critical viewer. After the successful rescue, Han Solo states "I've got to hand it to you kid, you were pretty good out there." This comment may not seem important, but in fact it is extremely weighty. Up until this point, Han Solo did not believe in Luke's abilities. Solo was the last to believe in him, and this comment proves that now, even Han Solo believes that Luke Skywalker has the potential to be something.
The hero's crucial struggle comes in the form of Luke Skywalker's final battle against the leviathan, his father, Darth Vader. The crucial struggle is defined as the hero's confrontation of evil. This statement holds exact truth in Return of the Jedi. Luke's struggle is against the very essence of evil itself, the "Dark side". Everything evil in the galaxy is thrown at Luke during this final confrontation, and only an individual of heroic proportions may resist these evils. The crucial struggle is unavoidable. It is a conflict which cannot be sidestepped, under any circumstances. This is evidenced when Darth Vader tells Luke, "You underestimate the power of the dark side.
