When Goodman Brown goes into the forest, what does he learn? What does he fail to learn?

When Goodman Brown goes into the forest that night, he learns that people that he assumed were model Christians were corruptible by the Devil and sin.  The Devil starts telling Goodman Brown of the people that he was able to corrupt, and the Devil starts with characters that Brown might think could be susceptible to sin.    

"I helped your grandfather, the constable, when he lashed the Quaker woman so smartly through the streets of Salem; and it was I that brought your father a pitch-pine knot, kindled at my own hearth, to set fire to an Indian village, in King Philip's war. 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 Brown doesn't deny the old man those claims.  Brown is a bit shocked, but he is more shocked that those people didn't talk about it.  

"If it be as thou sayest," replied Goodman Brown, "I marvel they never spoke of these matters; or, verily, I marvel not, seeing that the least rumor of the sort would have driven them from New England."

Next, the Devil begins showing Goodman Brown that he has been able to corrupt even the most incorruptible people.  Goodman Brown is shown that the minister and Goody Cloyse have had dealings with the Devil.  Goodman Brown is shocked, but he still fiercely hangs on to the fact that his wife, Faith, is still entirely pure.  Unfortunately, Goodman Brown learns that she too has fallen to temptation and sin.  The story ends with Goodman Brown being a broken and suspicious man.  He pulls away from every person in his town, and he even shuns his own wife.  

What Goodman Brown fails to learn and understand is that every person that the Devil shows him probably isn't a full Devil worshiper.  The Devil is showing Goodman Brown people that have fallen victim to his temptation and sin at some point.  Those people still struggle with sin, but that doesn't make them Satan's soldiers.  Goodman Brown fails to understand that every person is equally susceptible to temptation, and not everybody is capable of always resisting.  Brown failed to apply his basic Christian understanding that human beings are fallen and sinful creatures that are in constant need of forgiveness from God and each other.