The theme of “Young Goodman Brown” is the fragility of human spirituality.

Nathaniel Hawthorne frames his short story as an allegory, and the names of the two main characters, Goodman and Faith, immediately reinforce the religious undertone. Hawthorne uses seventeenth-century Salem, Massachusetts as the setting for many of his works, which allows him to critique the Puritans and specifically their teaching of predestination.

After young Goodman Brown encounters the devil and the seemingly dark religious event in the woods, his religious beliefs are clearly shaken. The woods are described as dark and unfamiliar, which ultimately confuses Goodman, as he cannot determine if the events in the woods are reality or a dream state.

This mirrors the relationship between the pious and their beliefs, as deeply religious people must submit to a profound lack of understanding. In the woods, Goodman witnesses the ostensibly religious townsfolk participating in a blasphemous ceremony. This one night in the woods forces Goodman to lose his faith—literally, in the fact that his relationship is forever changed with his wife, and figuratively, in that he cannot submit to the religious doctrine without questioning it.

Goodman lives the rest of his life unable to truly connect and trust those around him due to his night in the woods, which Hawthorne uses at the crux of his critique on the Puritans, which is that any religion so strict in its adherence as to not allow natural questioning is not viable.

"Young Goodman Brown" is focused on the concept of sin and how widely ingrained it is within the human condition. The story opens with Brown about to set out into the forest (and even early in the text, Hawthorne reveals that Brown's journey was made with an "evil purpose"), eventually running into the Devil, and being led to a Witches' Sabbath.

Within its pages, Brown has his eyes opened as to the reality of sin and of humanity's propensity towards it. Indeed, Hawthorne describes the scene of the Witches' Sabbath as one where the most respected members of the community and the most wretched, the saints and sinners, are all intermingled.

Additionally, I would say this is a story about a loss of faith. (Indeed, it's not by accident that his wife, who emerges as the pillar upon which his trust in the goodness of others tends to rest, is herself named Faith.) Ultimately, in the story of "Young Goodman Brown," Brown is shown a vision which reveals to him knowledge concerning the darker side of human nature, and he believes so strongly in the truth of that vision that he loses all trust in other people and can only see evil in the world around him.

Goodman Brown knows that he goes into the forest for evil purposes; this is why he doesn't want to tell his wife, Faith, where he is going or why. He thinks, "Methought, as she spoke, there was trouble in her face, as if a dream had warned her what work is to be done to-night." He wonders if she has some idea that he is leaving for some dark purpose. However, he decides to go anyway and vows that "after this one night, [he'll] cling to her skirts and follow her to Heaven." But this isn't really how faith works. Faith, the character, is also symbolic of faith in general: a belief in, trust in, and loyalty to God. It isn't enough to simply believe in God; one must also be loyal to God, and Goodman Brown's plan—to have one last night of sin and then ride Faith's skirts to goodness—is not how it works. Faith takes work, not rest. Therefore, Brown's unfortunate choice illuminates the theme that in choosing to be disloyal to God, no matter how short-lived one intends that disloyalty to be, one forsakes God.

“Young Goodman Brown” by Nathaniel Hawthorne is a story meant to leave its readers with more questions than answers. The story may be an allegory or may simply be the memory of a dream. The "good man" leaves behind "faith" to travel into a dark forest at night to attend some sort of service associated with the Devil. He is ashamed to be seen by fellow villagers, many renowned for their piety, including the woman who taught him his catechism, members of his family, notable preachers, and even his own wife Faith.

These encounters suggest a major theme of the story,  that the impulse towards evil is universal, lurking in even the most pious and good people, and that one cannot judge people truly based on one's own perceptions, because one cannot see into actions they do in private or into their hearts. 

The main message of the story Young Goodman Brown, by Nathaniel Hawthorne, is the fight between good and evil under the scope of detouring from one's faith and succumbing to the evils of life.

Throughout the story, Hawthorne exposes the natural weakness of human nature, and the vulnerability of the human soul to fall into temptation: Nobody, not even the very GOOD-man brown, is excluded from becoming exposed to evil. This is despite of what you deem to be your faith, or in what practices you incur to avoid falling into sin. The story clearly shows that religion, spirituality and a life of righteousness does not preclude complete protection from evil. Evil is everywhere, and anybody could become its victim.

Just like young Goodman Brown left his wife, Faith, and embarked in the dark road through the forest, individuals can deviate and end up in a dark road away from a life of righteousness. 

What might be the thesis of "Young Goodman Brown"?

"Young Goodman Brown" is a key example of Hawthorne's examination of the conflicts between good and evil that -- he believes -- are always at work in the human psyche.  There are several ways that you could state the theme/thesis of this story --

a) Wickedness exists in every human;

b) Even the people who appear to be models of goodness and propriety have secrets to hide;

c) Nobody is exactly as they appear.

The name of the main character, "Young GOODMAN Brown," is a tip-off to the fact that this story is going to be about morality.  "Goodman" is not so much the character's name as it is a common form of address in colonial times.  Nevertheless, it raises the question whether the young man is truly as "good" as he may seem. 

More in question is his wife, who is first introduced as "Faith, as the wife was aptly named."  Right from the start, Hawthorne sets up his two protagonists as model citizens, in what was a very religious community. 

On that night, Goodman Brown leaves his wife to go walking in the woods, meets a mysterious stranger (suggested to be the Devil), and ultimately sees a group of otherwise upstanding women from the town engaged in wild dancing that appears to be driven by witchcraft and devil worship.  Goodman Brown is horrified to see his wife Faith as part of the group. 

In the concluding paragraphs, we see that this incident, which may or may not have just been a dream, has the effect of shaking all of Goodman Brown's beliefs about the people around him.  Just as his beliefs are shaken, Hawthorne intends to raise questions for the reader as well.  Can we trust the trustworthy?  Can we believe what we think we believe?  Are good people as good as they seem, or do they have hidden evils?  Which is the reality, and which is the dream?

For an excellent overview, see the enotes.com reference on Young Goodman Brown at the source noted below.

What is the theme of Nathaniel Hawthorne's short story "Young Goodman Brown"?

Hawthorne discusses several topics in his short story/allegory "Young Goodman Brown"--alienation from society; destructive effects of guilt; women whose lives are ruined by the folly of men--I would argue that the over-arching theme of the story is the destructive effect of Puritanism on the individual, especially the concept that all men (and women, of course) are inherently sinful from birth.

Goodman Brown, an average Puritan young married man in Salem, Massachusetts, has a dream in which he decides to visit the "dark side," a probably unconscious desire to rebel against the restrictive, harsh belief system in which has lived.  He has been taught, among other things, that mankind is born in sin and is subject to temptation by Satan in many ways.  He also knows that he is predestined to go to Heaven or Hell and that, for the most part, what he does in this life will not change his final destination after he dies.

After having experiences in the forest in which his Puritan world is turned upside down--he meets Satan; he learns that very upright Puritan leaders in his life are Satan's converts; his own wife, Faith, who represents faith, is also on her way to join Satan's devotees--Young Goodman Brown completely loses his own faith,

'My Faith is gone!" cried he after one stupefied moment.  'There is no good on earth, and sin is but a name.  Come, devil; for to thee is this world given.'

He hurries to Satan's ceremony in the forest, and, during the ceremony, he tries to save Faith by telling her to "look up to heaven, and resist the wicked one," but he is unable to save himself from the belief that all men and women--no matter how righteous they may appear--are not just sinful but in league with Satan.

Goodman Brown, after this experience, is "a stern, a sad, a darkly meditative" man who died in "gloom."  His puritan belief system, which requires him to believe that he and everyone around him are sinful, blocks out the evidence of his eyes and experience that there are good people all around him (Faith, Goody Cloyse, Deacon Gookin).  Instead, his very strict interpretation of the Puritan belief system leads him to conclude that there is nothing good under the sun.