One of the most important messages of the play is that we must challenge corrupt authority and speak out against injustice, no matter what it costs us personally.
Abigail Williams herself tells John Proctor in act 1 that the girls were "only sportin'" when they danced in the woods. She says, quite clearly, that Betty's "not witched" and that the young girl is only afraid of her father's response. However, it takes weeks before John Proctor is willing to speak out against Abigail, and he only does so after his wife has been arrested for witchcraft. By then, it is simply too late. His motives appear to be colored by his desire only to free his wife, and the court has already acted on the girls' accusations with many others. Proctor had evidently not wanted to sacrifice his reputation by explaining his history with Abigail, and it eventually cost him almost everything.
Reverend Hale is another example of what happens when we don't speak up soon enough and when we don't try hard enough to topple corrupt authority or stand against injustice. Though Hale has no doubt of Rebecca Nurse's innocence, he tries to trust the court to exonerate her rather than speaking against the girls who accused her, and it ends up costing Rebecca her life and Hale his clear conscience. Later, when he does identify the court as corrupt, he simply quits it and leaves rather than staying and trying to work toward justice for the accused. Then, in act 4, he returns because he feels that there is "blood on [his] head" and that he is partially to blame for the innocent lives lost.