At what precise moment does Goodman Brown lose his faith?
Although Goodman Brown cries that his Faith is gone when he sees her pink ribbons, he later asks, "'But, where is Faith?'" Further, "hope came into his heart" at the witches' Sabbath. Brown and Faith are brought forth as "converts," and when they are brought together at the front of the unholy congregation, Goodman Brown looks at his wife and cries, "'Faith! [....] Look up to Heaven, and resist the Wicked One!"
To my mind, his hope of seeing Faith and then his instruction to her to resist the Devil indicates that he hasn't yet lost his faith yet, at least not fully. At this point, however, Brown doesn't know what happens to his wife: when he orders her to resist, he is immediately transported to a calm and quiet night, and she is gone. Then, the next morning when he returns to town, he "look[s] sternly and sadly into her face, and pass[es] on without a greeting." For the remainder of his sad life, he "turn[s] away" from his Faith, and this indicates that he lost it in that moment when he ordered her to resist and knows not what she does.
Goodman Brown appears to lose his faith after the eerie cloud has passed overhead and he sees pink ribbons falling from the sky. He exclaims at that point, "'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.'" Then he tears through the forest becoming a part of the evil scene. When he reaches the devil's meeting, however, he has one last gleam of hope, and appeals to Faith (the symbol representing his own faith) to resist the devil. The mysterious scene ends and Goodman Brown does not know what happened to Faith, but his own lack of faith is evidenced by his despairing behavior for the rest of his life.
He looses his faith when he realizes that his Faith (his new wife) is going to the Dark meeting as well. THat's when he realizes that maybe what he thought is good is now gone.
The key element in 'Young Goodman Brown' is the story's ambiguity. There really is no 'precise moment' when Brown loses his faith, because he arguably does not truly have any at the start of the narrative. From the moment Goodman Brown 'crosses the threshold' away from Faith at the start of the narrative, he has entered the ambiguous, dream-like realm of religious and moral questioning. Brown questions his own faith but also that of his community in Salem, and perhaps even the entire Puritan community. The devilish figure's mention of his ancestor's violent acts (there is a reference to witch burning, and the whipping of another heretic) clue us into the features of Brown's family and community that might trouble him into deep religious doubt. That up until the very end Brown implores his wife Faith to "Look up Heaven and resist the Wicked One!" is evidence of this being a tale of a faith tainted from its conception and not a pure faith lost at one precise moment.