This excerpt, from Rita K. Gollin 's "Ethan Brand's Homecoming" published in New Essays on Hawthorne 's Major Tales, edited by Millicent Bell, explains Brand's quest.

Ethan's quest was not for fame or fortune but knowledge of the Unpardonable Sin, a sin so heinous that it was beyond even God's mercy to pardon it. That search constitutes Ethan's identity and his legendary fame. He returns to the point where his search originated, his only home, a success in his own terms despite a few eruptions of doubt; and that very success also defines his human failure, his separation from all human connection except as a detached puppetmaster (97-98). (courtesy of Cambridge University Press)