Father Mapple is the clergyman at the little New Bedford Whaleman’s Chapel that Ishmael and Queequeg attend before setting out for Nantucket and their whaling voyage.
There are two main things you want to notice about Father Mapple. The first is that he’s a former whaleman himself who can’t seem to let go of the lifestyle. He preaches from a strange pulpit that looks like a bow of a ship, which he reaches by climbing up a ship’s ladder. Behind him is a painting of a ship in a storm observed by an angel looking down from the clouds.
The second thing to notice is that Father Mapple tells us the moral of Moby-Dick (to the extent that it has any moral) before we’ve even gotten to Nantucket. Preaching on the story of Jonah and the whale, Father Mapple explains that Jonah couldn’t obey God because he couldn’t disobey himself. If that’s not a lesson for Captain Ahab (who, unfortunately, doesn’t hear this sermon), we don’t know what is.