As Brown and Satan are walking through the forest, the first person they encounter is Goody Cloyse:
. . . a very pious and exemplary dame who had taught him [Brown] his catechism in youth, and was still his moral and spiritual adviser, jointly with the minister and Deacon Gookin.
A few lines later, we learn that Goody Cloyse not only recognizes the devil but also that he is "her old friend." The significance of meeting Goody Cloyse, who happens to be an old friend of Satan, is that Goody Cloyse was Young Goodman Brown's religion ("catechism") teacher when he was a boy and, more important, remains his "spiritual adviser." She was, in effect, responsible for teaching him the tenets of the Puritan's religious beliefs and guiding him along the path of conventional Puritan behavior. If Goodman Brown is still uncertain about the pervasiveness of evil in his society (actually, his mind), the fact that his trusted spirtual guide is an acquaintance of Satan should begin to convince him that he is surrounded by evil.
If, as is likely, Goodman Brown is on this journey with Satan to confirm his belief that evil permeates his life and the town in which he lives, there is hardly a more powerful confirmation than meeting someone he thought was a paragon of Puritan virtue on her way to a satanic ceremony--in fact, Goody Cloyse says she is there because "they tell me there is a nice young man to taken into communion tonight." From Goodman Brown's viewpoint, his Puritan world has just turned upside down.