Why is Brown surprised that the man he met knows his grandfather?

During
Goodman Brown's journey into the forest to participate in the Black Mass, he comes across a man
who resembles his father and the devil. The mysterious fellow traveler encourages Goodman Brown
to follow him on the path into the woods, but Goodman hesitates. He then comments to the devil
that his father and grandfather have never dared to travel into the forest with such evil
intentions. Goodman goes on to mention that he comes from a long line of worthy, honest
Christians and believes that he is the first man in his family to embark on the wicked
journey.

The devil responds by telling Goodman Brown that he is well
acquainted with his family. According to the devil, he helped Goodman's grandfather mercilessly
lash a Quaker woman through the streets of Salem. The devil also says that he brought Goodman's
father the "pitch-pine knot," which he used to burn an Indian village during King
Phillip's War. The devil goes on to say,

They were my good
friends, both; and many a pleasant walk have we had along this path, and returned merrily after
midnight. I would fain be friends with you, for their sake.

Goodman is astonished by this information because his grandfather and father were
celebrated, respected Christians with immaculate reputations. Brown then says,

We are a people of prayer, and good works to boot, and abide no such
wickedness.

Essentially, Goodman Brown is extremely naive
and held his family members in high regard. Since his grandfather and father appeared to be
righteous Christians, he could not fathom that they would engage in such wicked behavior or
participate in the Black Mass. One of the themes explored throughout the story concerns
humanity's collective wickedness, which does not exclude outwardly righteous individuals like
Brown's grandfather and father.