Miller gives an exposition about Parris, a brief overview of Salem, and some root causes of the events to follow in the play. He notes that the people of Salem were among those who came to the New World to develop/discover a "New Jerusalem." They tried to convert the Native Americans and were largely unsuccessful. This made them more defiant and single-minded. Although this was to be a pure, Christian culture, they were quite judgmental to the point of persecuting those who did not strictly follow their religious views and practices. So, these people were so steeped in their religious views and in the belief that they were fundamentally righteous, that they would invoke that religious superiority to justify anything they did: good and bad.
When it became "patriotic and holy" to accuse someone of witchcraft, people would do so but with other intents. Since it was acceptable and even applauded (in this strictly religious society) to accuse one of witchcraft, some people seized this opportunity to settle personal scores. In other words, if you wanted revenge, accuse your opponent of witchcraft and the religious leaders of the community would applaud you for it. It would be as if you were doing God's work; thus, it would appear that you elevated your problem to a divine or heavenly level:
. . . bickering over boundaries and deeds, could now be elevated to the arena of morality; one could cry witch against one’s neighbor and feel perfectly justified in the bargain. Old scores could be settled on a plane of heavenly combat between Lucifer and the Lord; suspicions and the envy of the miserable toward the happy could and did burst out in the general revenge.