how does the fellow traveler undermine goodman browns' faith?
The traveler, the devil, shows Brown that humans have more in common with evil than with goodness. Notice how as Brown begins his journey, he believes he is destined for heaven. However, the farther the devil lures him into the woods, the more he undermines Brown's faith. Whether or not Brown's relatives really were in league with the devil is never clear, but the devil just plants enough seeds of doubt in Brown's mind to shake his faith, which he never really recovers, for he is never able to look at his fellow citizens of Salem the same ever again.
In fact, the devil so completely undermines Brown's faith that he reduces him to the most frightening presence in the woods, despite the devil himself and all those gathered at the black mass.
The companion, the devil in disguise, slowly chips away at his faith by revealing that the family and fiends that Brown knew were not what he thought. As Goodman Brown chides himself for taking this walk, the devil tells him that he has a long history with the Brown family.
Every person they encounter is acquainted with the devil . The way in which the devil reveals this information destroys Brown's faith. In the end, Brown realizes he never really knew any of them.