I think the previous contributor has already given a very strong answer to this question. In Brown's time in the woods, he gains insight into the pervasiveness of human sin. He learns that the people around him, people who he had assumed were good Christians, were in fact not quite so pure and righteous as he had previously assumed.
That being said, there are criticisms one can level about Brown, especially when viewed from within the Christian tradition. One of the most significant questions is this: why does he place so much trust in the vision to begin with? After all, to put his faith in that vision ultimately entails putting his faith in the Devil. From a Christian perspective, this is obviously a deeply problematic thing to do.
In addition, I would note that, from the very beginning of the story, Brown is portrayed as possessing a very simplistic, black-and-white view on human nature. He idolizes his wife, casting her as a paragon of goodness—referring to her as "a blessed angel...
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