What is the subject of Mr. Dimmesdale's Election Sermon? The author speaks of "profound and continued undertone" in that sermon. What is the nature of the undertone?

Dimmesdale has finally come clean. The contentment he talks of is his public confession. The poison that has rankled his body and soul are purged and he dies soon after.

His subject in the sermon is the relationship between God and the communities of mankind.

After his sermon is over, Dimmesdale leaves the meeting hall and goes to the scaffold where he proceeds to call to Hester and Pearl and confesses.