In Nathaniel Hawthorne's story, Goodman Brown experiences a total crisis of faith. While the authors leaves unclear whether Brown actually saw what he thought he saw, or whether his own guilt got the best of him is in some ways irrelevant. Part of the story's power, as well as its relevance to its colonial Salem setting, inheres in that very ambiguity.
Goodman sets off into the forest on an errand he does not disclose to his wife, but says he'll be right back. In the course of his adventure, he sees everyone whose good character he valued doing abominable things or represented as diabolical symbols. Even his own grandfather and his wife are not immune. Perhaps even worse, he sees himself fully participating in these evil acts.
At a certain point, it all vanishes. Was it all real? Did he pass out and come to after they left? Or was everything an illusion or a dream? Regardless, Goodman can never see the world the same way again. To him, the reality of what he saw is undeniable; all the others are flawed and sinful. The alternative—that his own dark mind sees sin everywhere, whether present or not—is perhaps too unpleasant for him to contemplate.