Sometimes, we can't rely on external sources to provide justice for us. In The Crucible, relying on the courts for true justice did not work out too well for many people in Salem. In act three, as John, Francis, Giles and Mary come to the courts with firm proof and logical rationale regarding the cases in the court at the time, the judges refused to dispense equal and fair justice. Innocent people were hanged, and many others unfairly jailed. The courts were supposed to enact justice and did not. The message was clear: do not rely on the courts for justice.
Also, do not rely on others to be just. If everyone in the play had been just, no accusations would have been made. In fact, many of the accusations made were people's attempts to thwart justice's own demands of them. Justice would demand that Abby and the girls be punished for dancing and casting spells; to avoid that justice, they lied and accused others. If anyone in Salem counted on people being kind, fair, and just, they were disappointed. Instead, fear, paranoia, panic, betrayal, jealousy and greed reigned supreme.
The only true justice that occurs in the play is the justice that people are able to bring to their own hearts. At the end of the play, John finally is at peace with himself. He feels like he has finally paid for his sins, and done enough good deeds to earn redemption. He walks to the gallows a free man, because he brought his own justice to his life. He paid for his sins, he righted his wrongs. He was justified and at peace at the end.
The major message is to not rely on others, or institutions for justice; it will often end in disappointment and wrongdoing. Instead, find your own way. I hope that helped; good luck!