We don't know if Goodman Brown's journey into the woods is real or a dream. However, this distinction is irrelevant because either Brown literally saw the townspeople (including religious leaders and his wife) at a Black Mass or it is symbolic of his discovery that they all are capable of, or have committed, some evil. Faith symbolizes the good of humanity and Brown's faith in that goodness. When he leaves to go into the woods, he abandons that faith and then discovers that the goodness of humanity also contains evil. Even though he returns, he has permanently lost some of that faith and, from this revelation, he will see the duplicitous nature of humanity (good/evil) for the rest of his life. But he will dwell on evil too heavily.
Whether a dream or a reality, Brown never knows if Faith made a covenant with the devil:
'Faith! Faith!' cried the husband. 'Look up to Heaven, and resist the Wicked One!'
Whether Faith obeyed, he knew not. Hardly had he spoken, when he found himself amid calm night and solitude, listening to a roar of the wind, which died heavily away through the forest.
This story is marked by Hawthorne's interest in the very strict religious culture of the Puritans. Brown becomes similarly fanatical and he can not reconcile the fact that people are imperfect and capable of sin. After his journey/dream, any time he sees someone doing a good deed, he deems it hypocritical.
That being said, it isn't clear whether or not Faith has committed some terrible sin. What is clear is that Brown concludes that everyone sins and he lets this prevent him from seeing goodness in people. Brown has given up. Faith has not, as we can see when she skips through the street to greet Brown when he returns. It is Brown's judgmental descent into despair that might be considered a literal sin. All other indications of people sinning (whether literally at the Black Mass or symbolically sinning in life) are generalizations of those people made from Brown's perspective. The fact that Brown didn't see whether or not Faith resisted the devil is more telling of Brown's own uncertainty of his own faith than it is about Faith herself.