In Chapter XX of "The Scarlet Letter," Hawthorne remarks, "No man, for any considerable period, can wear one face to himself, and another to the multitude, without finally getting bewildered as to which may be the true." Dimmesdale wears these two faces: He returns from the forest encounter with Hester "transformed" as Hawthorne writes. Dimmesdale has three urges to rebel against his false nature by insulting or lying to townspeople; he is tempted to sin. When he returns to his house, he sees the Election Sermon. It is though another man has written this sermon as he now has more knowledge, a "bitter knowledge," the knowledge that he has been deluding himself.
In the words of Emily Dickinson, when Dimmesdale does tell the truth he "tells it slant" not because the people will not understand, but because he has not the integrity to confess his sin as adultery; he is too weak to defend Hester and can only stand on the scaffold in the cover of night. He feigns that his illness is only a physical illness, and he misleads Hester into believing that he will leave America with her, even deluding himself into this belief knowing he cannot bring himself to escape. He continues to wear "two faces" and is, therefore, a hypocrite.
Everyone in Puritan time was considered a sinner. The priests in the church held the power they did because they were chosen by God and see as sin free. Even if Dimmesdale confessed to being the "worst of the sinners", very few in the town would actually believe him.
Dimmesdale also doesn't admit to his specific crime--adultery with Hester. He allows her to suffer alone, to be ostracized by the community while he is still able to live in town and go about his life (though not without some physical pain). In the beginning of the book, he allows Hester to stand alone on the scaffold, and he takes part in the attempts to make her reveal the name of the man who fathered Pearl.Dimesdale's sin also allows him to be more passionate and heart-felt in his sermons against sin. This, in turn, makes parishioners love and respect him even more, though they are respecting and heeding a the word of a man who cannot take his own advice.