A major irony of "Young Goodman Brown" is that while Goodman Brown tries so hard to hide his sins from the members of his community in Salem, all those people actually turn out to be sinners themselves.
In Puritan culture, appearing virtuous to others was crucial: any whiff of sin, no matter how flimsy the evidence, could get someone ostracized or worse. (Nineteen innocent people were executed during the Salem Witch Trials, around the time when this story is set.) For this reason, Goodman Brown is terrified throughout the story at the possibility that someone in his community might see him in the woods and (correctly) assume that he's there to do something sinful. The possibility of being perceived as sinful seems to trouble him much more than the actual fact that he's sinning.
The irony here is that the very people he's trying to hide his sin from all appear in the woods themselves, showing that they're no better than he is. Every person Goodman Brown admires or respects turns out to be in league with the devil: his father, his minister, the woman who taught his Catechism, his wife.
Of course, there's another possible interpretation here: that Goodman Brown simply dreamed that all of these people were secretly sinners, and his dream felt so real that he could never trust any of them again. But if that's the case, it still shows how deeply convinced Goodman Brown is that sin lurks in the heart of every person (including himself). His insistence on going through the motions of family and religion and community for the rest of his life, despite not believing in any of it, is ironic.
