To begin with, we simply do not know what Brown saw. Everything in the story is ambiguous (there are references to this in other enotes posts you might want to check). One of the important thing that happens to Brown is that he has an experience (whether real or just a dream) of many of the people he had previous considered to be "good" in the town involved in some mysterious ceremony (a Black Mas?) that is happening in the woods. Not only are the obviously "evil" there, but there are "reputable pious people, chaste [or so he thought] dames and dewey virgins, and [even] the Revered pastor." Brown cannot understand how people he held in high esteem, people he thought were above evil, and just as much a part of it as people who were clearly and openly sinners.
This is what he thinks he saw ... there is no certainty that he saw anything. But it's not important; the sight turns him into a cynic who can no longer accept the complexity/ambiguity that is part of life.