what lesson does father maple strive to teach the whale men in the chapel who are about to embark on their journeys

Father Mapple's sermon continues to set the tone for the novel, making the obvious comparison between the story of Jonah and the whale and the impending conflict between Captain Ahab and Moby Dick. Several chapters before the character of Ahab is even introduced, Melville prepares the reader for a comparison between Ahab and Jonah, for both characters flout God through their arrogance and disobedience against God. The story that Father Mapple recounts of Jonah is a long detail of hardship and pain, thus foreshadowing the difficult voyage of the Pequod, and constructs Jonah to be a complex man gripped by terrors in his soul, another important comparison between Jonah and Captain Ahab.

Other than the comparisons between Jonah and the as yet not introduced Ahab, Father Mapple's sermon is also significant for its final instruction to preach the truth in the face of falsehood, even if that truth may be very difficult to take. This part of the sermon relates most directly to Ishmael himself, as the narrator of the novel. This suggests that Ishmael must tell some unspeakable and harsh truth about his voyage with Captain Ahab, no matter how unpleasant that truth may be.

In general, Father Mapple's sermon continues to imbue the novel with a striking religious theme. This religious atmosphere is one with a particular emphasis on pain and suffering which leads to redemption; it is a harsh and brutal form of spirituality the conforms to the brutal setting and nature of the novel. Melville relates spirituality to intense struggle, thus giving the voyage that will be the heart of the novel a larger significance.