The devil promises that if Goodman Brown is baptized, he will be able to know the secret evil within each member of the human race - he will see "where crime has been committed, and shall exult to behold the whole earth one stain of guilt". As Goodman Brown stands waiting to be baptized with his new wife Faith, the horrible thought that they should see the blackness within each other causes him to refuse the lurid sacrament, and at the last minute he exhorts Faith to join him and "look up to heaven, and resist the wicked one".
I think it can be argued the the devil's promise is a fitting reward for Goodman Brown because the reason he chose to leave his Faith and walk awhile with the devil is his curiosity. Goodman Brown wanted to know about the dark side of human nature, and ironically, that knowledge is exactly what he received. Critics assert that Hawthorne, by shrouding Goodman's life with gloom even though he did not go so far as to receive the devil's baptism, is commenting on the unforgiving nature of Puritan religion. I wonder also if he perhaps may be making a statement about the danger of yielding even a little bit to the temptation of "wanting to know" - that innocence can be lost even by the mere experiencing of evil, even if one does not succumb to it.