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.