The Puritans felt that the forest primeval, or
...the virgin forest was the Devil's last preserve, his home base and the citadel of his final stand...to the best of their knowledge the American forest was the last place on earth that was not paying homage to God. (Act I)
The edge of the forest is close to the Puritan settlement, and it is mysterious to the residents of Salem. Dark and threatening, the forest holds the wild and sometimes very savage Indians who maraud the settlements from time to time.
The forest is a place of mystery that lends itself to superstition and fear. Since the Puritans perceive the forest as the place of the Devil, when pretty Abigail informs her uncle, the Reverend Parris, that the rumor of witchcraft is about, Parris asks her if she has "trafficked with spirits" in the forest. He insists that he must know because if he is not informed, his enemies will know, and they will surely try to ruin him.
Additionally, the forest provides the Devil the environment needed in order "to whip men into surrender" to a particular place. Puritans feared the confines of the forest where the Devil could exert influence in their lives.
The Puritans believed that the devil was alive and well in the new country in which they had just recently settled. The forest was a great place for the devil to hide! In order to survive, the Puritan settlements like Salem had to stick together. They needed each other to stay alive in the new, wild country they came to for religious freedom. Everyone had to pitch in to make the community successful and to perpetuate their religion. In addition, and unfortunately, the "savage," heathen Native Americans also lived in the wilderness. The Puritans feared them as well. They considered them subjects of the devil because of their "uncivilized" ways.
To keep the communities together and continue to make them successful, a common enemy was a good way to bring them together. There was no one better to have than the devil as the basis for their fear and to keep them united towards a common cause, the practicing of their religion.