The Crucible gives the reader the opportunity to observe the concept that perception is truth. The idea that, the actual truth has less value than the perceived truth. For example, because of the hysteria caused by the Puritans inability to accept that bad things happen, such as infant and child deaths, cattle or livestock, or pig death or sickness that can not be explained, they decided to invent a cause "witchcraft".
The black magic was performed by people that the town elders or administrators did not like or understand. Or those who engaged in pleasures of the flesh which were strictly forbidden by the Puritans.
So a group mindset, or "Groupthink" is developed based on perception rather than truth. This runs the entire conflict and plot of the play. Innocent people are put to death for witchcraft to satisfy the perception that evil is being eliminated from the town. When in fact, the very act of putting innocent people to death is EVIL.
One of the main things that the audience/reader is faced with is the power of fear. Fear can lead normally good people to do horrific acts, especially if religion is involved. Let's start with the girls. They were caught dancing in the woods with Tituba. According to the Puritan beliefs, dancing is a sin and the work of the devil. The girls knew what punishment they would face (whipping, being put in the stocks, public ridicule/scorn), so they followed Abigail's lead and said that they'd been possessed by a devil. Abigail took it further, though, to use it to her advantage. She bagan to claim that certain people cast spells on her to make her do "bad things". When the trials started, Abigail became the star "witness" and pointed fingers at anyone who got in the way of what she wanted. That included Elizabeth Proctor (the wife of Abigail's former lover, John Proctor). What began as acting out of fear to avoid punishment became a powerful tool of revenge for Abigail. As for the people of Salem, their reaction to the girls' accusations was to rid their town of the devil's influence. They became fearful of one another and began to point fingers at each other, blaming innocent people for things that simply went wrong in their lives. They began to claim that misfortune was the act of the devil, and so they hanged people to rid themselves of his influence.