How is Hawthorne advancing his theme of the difference between revealed and secret sin? In chapter 17.

Hester reveals that Chillingworth is her husband, and it is clear that Dimmesdale has crossed the point of no return. He has withheld confession long enough to have to die for his sin, as though he has traded his own soul, his own daughter, his own love all in order to preserve a semblance of self-preservation. And in return, he has lost his self-respect and will to live. Had Dimmesdale confessed earlier, there might have been the possibility not only for redemption, but also to start anew by leaving town with Hester and his daughter. He believes the possibility still exists, but the crush of shame and guilt now deny him the option.

It is telling here that Dimmesdale, at first a portrait of unyielding stringency, ultimately forgives Hester. She, after all, has committed a terrible crime by not telling him that Chillingworth was her husband, but we sense that Dimmesdale takes responsibility for this as well. If he had had the courage to confess his sins, then he never would have fallen into the power of such evil. Finally, Dimmesdale is beginning to make amends, and he is coming to believe that he is the root of much evil, which means to him that upon his death, he will open up a path to a new life for everyone else.