The forest takes center stage in The Scarlet Letter as an enigmatic symbol of darkness, particularly, the darkness that lures within the souls of all humans.
As a setting, it is the place where anything goes, as nobody really knows what lurks beneath this complex, dense, and mysterious place. This very lack of knowledge about what really takes place in the forest leads to the myths that the villagers build about it; this place whose very abstruse nature sheds a supernatural light onto everything that lives in it.
All these giant trees and boulders of granite seemed intent on making a mystery of the course of this small brook; fearing, perhaps, that, with its never-ceasing loquacity, it should whisper tales out of the heart of the old forest whence it flowed, or mirror its revelations on the smooth surface of the pool.
Aside from its mystical interpretation within the novel, there is a very realistic rationale behind the fear that the forest instills. In reality, the forest is a hiding place for everyone and anyone who wishes to get away with something. We know that the town's witch, Mistress Hibbins, openly declares to Hester that she has done her works in the forest.
Another fact about the forest is that it is the living headquarters of the Native Americans. To the sanctimonious puritans, the "Indians" were the non-Christian, hell-bound savages who could kill them at any given time.
The forest is also the pathway used by the sailors to reach town after their return, still carrying with them the all the superstitions, rowdiness, and mystic energy of the sea. The sailors, rude and wild as it is evident during the Election Day sermon, constitute the other savages whose drunken ways could earn them the next victim from town. The forest is a potentially dangerous place because it can be used by people who want to hurt others and get away with it.
In the eyes of the villagers, the forest is also an enabling location. This is because it harbors subjects that share its main qualities: beings who are dark, shady, isolated, mysterious, unpredictable, and unholy. In reality, they are not far from the truth. The forest waitresses things that would otherwise suffer from public scorn, such as the conversations between Hester and Arthur, or the harassment from Chillingworth to Hester as he finds her with Pearl walking there.
In all, it is the unknown what the villagers fear the most. It is the uncertainty of life and death what leads them to trust Calvinism, in itself a doctrine based on uncertainty, with blinded eyes. It is their fear what ties them so fiercely to the strict discipline of their church for fear of what their "angry God" would do to them. This is why, out of all the symbols in the novel, the forest is the one that most effectively sets the tone, mood, and atmosphere of the story.