When Goodman Brown first meets the stranger in the forest—later revealed to be the devil—the story draws attention to his serpent-like staff, which is an allusion to the serpent in the Garden of Eden:
But the only thing about him, that could be fixed upon as remarkable, was his staff, which bore the likeness of a great black snake, so curiously wrought, that it might almost be seen to twist and wriggle itself like a living serpent. This, of course, must have been an ocular deception, assisted by the uncertain light.
In the Bible, the Book of Genesis describes a serpent luring Adam and Eve into eating the fruit of the tree of knowledge, which God forbade them to do. The serpent's influence leads to the fall of man in which Adam and Eve are expelled from the Garden of Eden, and their sin condemns all of humankind to be corrupted by evil. In other words, according to the Bible, the serpent is responsible for the greatest catastrophe in the history of humankind.
The stranger's serpent-like staff reveals his identity: this man is the devil himself. This is clear based on the biblical association between the serpent and the devil, and also based on the supernatural way in which the staff moves—this man seems to have supernatural powers that would be beyond any human being. Goodman Brown rationalizes the staff's uncanny motion as a trick of the light, but he crucially calls it an "oracular deception," which evokes one of Satan's monikers: the "great deceiver."
Based on this allusion to the biblical fall of man, readers can intuit that the man with the serpent staff is going to lead Goodman Brown into sin that will ruin his life, just like the serpent did to Adam and Eve.
