What lesson does Rev. Dimmesdale learn in chapter 20 of The Scarlet Letter?
It is a profound lesson about himself that the Reverend Arthur Dimmesdale learns after his forest visit with Hester and their child. For, while he has traversed the forest path after having decided with Hester that they will leave the Massachusetts colony and return to England, Dimmedale has felt a sense of release from his secret sin; this release is, however, a wicked one as he has been tempted to commit "wild wicked things" as he encounters various members of his congregation along the way. When he meets Mistress Hibbins, she recognizes in Dimmesdale his wicked temptations and laughs at his protestations that he has not go into the forest "to seek a potentate."
After this chance meeting with Mistress Hibbins, "the old-witch lady," the minister wonders,
"Have I then sold myself...to the fiend whom, if men say true, this yellow-starched and velveted old hag has chosen for her prince and master!"
Dimmesdale concludes that he has indeed "made a bargain very like it!" Tempted by the dream of escape, Dimmesdale has yielded himself to "what he knew was deadly sin." His encounter with Mistriss Hibbins has, thus, brought to light his ridicule of good and evil, his "unprovoked malignity" and "gratuitous desire of ill."
Returning home, Dimmesdale enters the room where he has been working on the Election Sermon. As he peruses the unfinished sermon, the minister perceives that now, after his forest visit, he is not the same man,
But he seemed to stand apart, and eye his former self with scorful, pitying, but half-envious curiosity. That self was gone.
Dimmesdale learns that another man has come from the forest; he is a wiser one, one with the awareness of "hidden mysteries" which the simplicity of his former self has not admitted. Arthur Dimmesdale fully recognizes his great sin--"A bitter kind of knowledge that!" as he admits to himself his great hypocrisy and guilt for which he knows he will be punished.
In chapter 20 of The Scarlet Letter we learn that Hester and Dimmesdale have decided to escape with little Pearl. This is a welcome change for Dimmesdale. He finally lets himself breathe after years of holding the guilt of seeing Hester pay for a crime he committed with her: Their adultery.
Now that Dimmesdale feels free from the prying eyes of the settlement, he begins to change. He even rejuvenates! The part of him that he has been concealing under a self-imposed punishment slowly begins to come out. Slowly, we begin to see the Dimmesdale that is capable of adultery and that hides many dark secrets in his heart,which he apparently controls with self-punishment and the self-righteousness of being a reverend. He feels like succumbing to temptations of all sorts, especially of saying sacrilegious things. This leads him to the conclusion- which is the lesson that he learns- that he has just submitted his soul to the devil, and that Hester may have led him to harm's way again. Encountering Mistress Hibbins does not make things any better, since he uses this as confirmation that Hester is taking him to the ways of evil. What he learns about all this is that he is bound to perdition if he does not fix the situation immediately. His basic lesson is that he has failed as a servant of god- again.
We could say that this would be the last time we see Dimmesdale thinking about Hester. In chapter 23 we see that he completely ignores Hester as he goes to the meeting house and that Hester can sense that he backed out of their plan. This should not come as a surprise to her but certainly the reader can agree that Dimmesdale is one of the weakest and most unreliable characters in literature.