Throughout the story, trees and wilderness are a motif that emphasizes the inevitability of sin.
At the beginning of the story, the town and wilderness are clearly differentiated—the town is a civilized place of light and virtue, whereas the dark forest surrounding the town is a place of evil, danger, and sin. This neat distinction between town and wilderness mirrors Goodman Brown's own beliefs about the world: that the people in his community are good, and that any evil in the world exists in the people and places he doesn't know, like the woods he's never before traversed.
When Goodman Brown initially ventures into the woods, he thinks he's simply indulging a perverse curiosity about sin and will be able to return to his old life in Salem as soon as he's out of the woods. He's not so concerned about his own sin of meeting the devil in the forest; instead, he's afraid that a stranger or "devilish Indian" might attack him. This again reflects his naive belief that evil can't be found in the things he knows (himself, his town) but must instead be located in the unknown.
However, as Goodman Brown gets deeper into the forest, his ideas about where evil is located begin to change. First, he begins to see people from his community—good, respectable people—going through the woods, likely for sinister purposes. This mingling of townspeople and wilderness makes Goodman Brown understand that evil isn't just located in the woods—it's also in Salem, the place he once believed was straightforwardly virtuous. When he sees his wife, Faith, at the demonic gathering in the heart of the wilderness, he loses his faith in God and in human goodness entirely. And for the sin of his curiosity, he then spends the rest of his life bitter and cynical, unable to enjoy his life or to love those around him, as he believes they're sinful.
In this way, Goodman Brown's journey into the wilderness changes him forever, and he cannot simply return to the innocent life he once lived in town. The evil of the wilderness is inside him (and the story implies that it always was, since he was compelled to follow the path into the woods even against his better judgment), but he cannot live peacefully with this knowledge.
The path Goodman Brown walks into the heart of the forest isn't simply a path: it's also a motif in the story that evokes the inevitability of sin.
The story associates the forest with evil, so it would make sense that the path—a sign of human civilization in the wilderness—would lead Goodman Brown away from sin. But it doesn't: the path leads Goodman Brown straight into the heart of the forest, where he is forever corrupted by evil. In this way, the path and the wilderness are not so different (since they both lead to sin), and this evokes a central message of the story: that human beings are just as corrupt as nature because, in fact, we are part of nature.
This dark aspect of human nature is reflected in Goodman Brown's inability to stop following the path deeper into the forest, even as he tries to muster the conviction to turn back to Salem. His compulsion to follow the path, regardless of how much he wants to turn around, reflects the human compulsion to sin. The path itself also seems to make turning back less possible as he continues on. At the outset, the path is big and clearly delineated from the wilderness; Goodman Brown could easily follow it back to Salem and renounce his sinful intentions. But as he continues on, the path grows fainter and less distinct from nature. Just before the story's climax, the story describes the path this way:
The road grew wilder and drearier, and more faintly traced, and vanished at length, leaving him in the heart of the dark wilderness, still rushing onward, with the instinct that guides mortal man to evil.
That Goodman Brown continues into the wilderness even after the path has disappeared shows that his impulse to sin comes from within him, not from anything external. One could conceivably see the path into the wilderness as a kind of temptation—it's proof that others have been there before him, and that he's not alone in his sinful inclinations, making them perhaps less frightening and more enticing. But once the path disappears, he's operating on "the instinct that guides mortal man to evil." He's no longer following a path, he's simply following his own heart. (Also notice that Hawthorne universalizes this, implying that all people have this instinct, not just Goodman Brown.)
