If you are referring to the main idea or the lesson to be taught from Young Goodman Brown you can conclude that Hawthorne wanted to send a message to those who call themselves "holier than thou".
Evil is a capacity that everyone has. Everybody has the potential of committing sin, to be led into temptation, and to turn into something they never thought they would.
In Young Goodman Brown, a man who always followed the right path is diverted by a strange man in a dark forest in which he encounters those very people whom he once considered "good" turned into creatures of evil. This, Hawthorne would argue, is what happens when you deviate from your faith (Faith is also the name of Goodman Brown's wife, whom he left to go into the forest).
Therefore, the morale of the story is that our souls can be corrupted no matter what, and that no matter how much you want to claim your kindness and purity. We are nevertheless human, and human only.