In Chapter 20 of The Scarlet Letter, why exactly does Mrs. Hibbins congratulate Dimmesdale?
Mistress Hibbins in this Puritan community is a figure who is well known for believing in the devil and is recognised as being somebody who goes to the forest in order to commune with him and practise witchcraft. When she therefore comes across Dimmesdale coming back from the location of the forest at night, as if he were trying to hide the fact, she comes to only one conclusion: that he has gone to the forest to meet with the devil too:
"So, Reverend Sir, you have made a visit into the forest," observed the witch-lady, nodding her high head-dress at him. "The next time, I pray you to allow me only a fair warning, and I shall be proud to bear you company. Without taking overmuch upon myself, my good word will go far towards gaining any strange gentleman a fair reception from yonder potentate you wot of!"
Mistress Hibbins therefore congratulates Dimmesdale on joining her "side" and trying to commune with evil. She clearly feels that winning such a person as the young Reverend who is so popular with his congregation to her side is a victory worth celebrating. Of course, we know that she has misinterpreted the situation, but ironicaly Hawthorne uses this event to challenge Dimmesdale's perception of what has happened with Hester and his plans for escape.