It's difficult to answer your question because Dimmesdale gives several sermons in the novel. The sermon that has the most recorded about it comes right after Chillingworth has moved into the same house as Dimmesdale and Hester has been allowed to keep Pearl. In that sermon, his subject is sin and he expresses the thought that he is the worst of sinners. Hawthorne makes it clear that Dimmesdale wanted to confess his sin of adultery during that sermon, but he never does. Ironically, people think of Dimmesdale as more of a saint after the sermon because they reason that only a saint could think of himself as such a sinner. Little do they realize that he is an adulterer and the father of Pearl.