what is the subject of the sermon by father maple

Father Mapple arises to the pulpit and pauses, then offers a prayer in which he states "I saw the opening maw of hell, / With endless pains and sorrows there; / Which none but they that feel can tell ­ / Oh, I was plunging to despair." He then gives a sermon that considers Jonah and the whale, wondering that the lesson of this tale is. He claims that it is a lesson to sinful men and to God, for Jonah flouts God with the sin of disobedience, thinking that a ship made by men will carry him into countries where God does not reign. Father Mapple recounts the terrors in Jonah's soul. The lesson of Jonah is to preach the Truth in the face of Falsehood, even if this truth may be appalling.