is.

By definition, a protagonist is a character around which the story revolves—who the story is mainly about. Hawthorne's tale is about Brown's journey into the forest one evening on an "errand."

The antagonist has to do with the story's conflict (though there may be more than one conflict), and is seen as the character "or force" that opposes the protagonist. In reading the story, one might first believe that the antagonist in the story is the old man who represents the Devil. His appearance frightens Brown, though the text notes that he is not surprised to see the "man" there.

“You are late, Goodman Brown,” said he. “The clock of the Old South was striking as I came through Boston, and that is full fifteen minutes agone.”

“Faith kept me back a while,” replied the young man, with a tremor in his voice, caused by the sudden appearance of his companion, though not wholly unexpected.

As Brown and the old man travel, we learn that his companion has known many of Brown's ancestors; as they continue on together, many of the most prominent citizens of this Puritan community are also in league with the Devil, heading to a Black Mass in the middle of the woods. Not once does Brown complain or attempt to leave the old man, suggesting that Brown is there by choice. However, when he sees his wife, Faith, at the gathering, he loses his trust in her goodness. In fact, it is after this that Brown loses trust in the virtuousness of mankind. This knowledge is what changes Brown.

The next morning young Goodman Brown comes slowly into the street of Salem village, staring around him like a bewildered man.

Brown perceives his neighbors in a new way: he sees "the good old minister" and "shrinks from him." Old Deacon Gookin is praying at his window, and Brown wonders,

What God doth the wizard pray to?

As he continues, Brown sees the woman he once revered:

Goody Cloyse, that excellent old Christian, stood in the early sunshine at her own lattice, catechizing a little girl who had brought her a pint of morning's milk. Goodman Brown snatched away the child as from the grasp of the fiend himself.

Finally, Brown sees his new wife, who he so loved before his visit to the woods. Now, he can only see her based upon his experience the night before, something he is not even sure actually happened. However, his lovely wife is now an "anathema." He walks past her without speaking. And from then on, he is a cynical, broken man.

Had Goodman Brown fallen asleep in the forest and only dreamed a wild dream of a witch-meeting?

Be it so if you will; but, alas! It was a dream of evil omen for young Goodman Brown. A stern, a sad, a darkly meditative, a distrustful, if not a desperate man did he become from the night of that fearful dream.

The question regarding who the antagonist is does not, as I see it, point to the Devil. The antagonist is Brown himself. In terms of the story's conflict, it is man vs. self. His lack of faith in others, perhaps in knowing he is not the man he wishes he could be, destroys his relationships with everyone he knows. His inability to accept imperfection in others and self, are what destroy Young Goodman Brown.

This would seem pretty straightforward, but if we look at “Young Goodman Brown” as a dream, it has a twist. First, put simply, Young Goodman Brown is the protagonist. You could also say that Faith is a second protagonist, especially since her name is a pun on Young Goodman Brown's own faith. Thus, his faith, a part of his own mind/soul, must necessarily be a part of himself. That pun aside, Brown himself is the protagonist. The devil is the antagonist. These characters could be real or part of Brown's dream, part of his own consciousness. It would seem that this is a dream vision. That being said, Brown still believes it fully because the next day, he has clearly lost faith in others. “A stern, a sad, a darkly meditative, a distrustful, if not a desperate man, did he become, from the night of that fearful dream.”

And here's the twist. In this case of this being a dream, you could say that Brown is the protagonist and the antagonist. This is a battle in his own mind. Once he becomes aware of, or suspects, that his fellow church-goers are sinners, he loses his faith in them and then in himself. He begins to believe that “Evil is the nature of mankind.” Brown struggles with his own faith and his own conscience.

Brown's inability to accept the potential for people to be bad and good, faithful but flawed, is Hawthorne's indictment of the extreme black and white philosophy of Puritan religious beliefs.  

allegorical

As an allegory "Young Goodman Brown" presents characters that are symbolic of human traits.  The names as symbols are obvious, the most obvious being that of Faith, whose innocence is symbolized by the pink ribbons which are seen a significantly religious three times:  as Goodman departs for the forest and his meeting with the dark forces of life, as he sees them in the forest "floating from the heavens," and as he returns home having "lost my Faith."

The title "goodman" in Puritan times was one given to socially beneath a gentleman, a title suggesting unworldliness and vulnerability.  The old man with a serpent-like staff represents the Devil; he resembles Goodman's grandfather and says that he is acquainted with Goodman's father who participated in the burning of an Indian village.  These references suggest that Goodman is not descended from tuly good men.

The names Goody Cloyse, the Minister, and Deacon Gookin suggest the hypocrisy that lurks behind the apparent good.  In fact, Goody Cloyse and Deacon Gookin were actual Puritans who pariticipated in the Salem Witchcraft Trial of 1692.

That Hawthorne meant for his story to be an allegory is evidenced not only by the use of these real names, but also by his belief that tales should be ones that teach moral lessons.

In one case, the protagonist and the antagonist are one and the same person--Young Goodman Brown. It is a psychological short story of Hawthorne's implying the the good and the propensity for bad are in each one of us. Young Goodman Brown has a crisis of faith and temptataion. He hides his ignonimous journey from his young wife Faith, choosing to confront the devil himself. It is hard for the reader to tell for certain, however, since this might be reality and it might be some nightmarish dream.

On another level you could say the antagonist is the devil or the old crone that meets and taunts Young Goodman Brown in the midnight forest. Since Young Goodman is the protagonist, he's the character that must go on a spiritual journey and he's the one who witnesses the Black Mass in the clearing. Goodman sees that even the town's most pious and holy are there. Even his wife appears and he is spiritually defeated. The antagonist might become in this case, everyone who Goodman saw as good but had some dark and evil secrets....  

An interesting question. The characters do play specific roles, and roles somewhat different from those in most fiction. Goodman Brown is like anyone. You could call him Everyman. He stands in for any person, anywhere, any time. He is an allegorical representation of the reader. His wife Faith stands for both faithful companions, like one's wife, and the characteristic or quality of faith. Therefore, when he leaves her to walk through the woods, he walks without faith. His pride leads him astray, and the other characters help him go astray. They representation temptation and/or false idols; they are the people he thought he knew and could trust.