What is the main theme in Herman Melville's novel Moby Dick?

Herman Melville dedicated his novel, Moby
Dick, to Nathaniel Hawthorne and wrote him, "I have written a wicked book,
and feel spotless as the lamb."  While there are several major themes in Melville's
great work, perhaps the central theme is that of the individual in conflict with nature
which brings into play Religion and God's role in the natural
world.

Melville
marked repeatedly verses from the book of Job, such as the verse in the fourteenth
chapter when Job asks his despairing question about a future life, "But man dieth and
wasteth away: yea, man giveth up the ghost, but where is he?"  Certainly, the
implications here of the white whale as a metaphor for the forces of nature and fate are
apparent. 

In the beginning of the novel, Father Mapple
gives a sermon that reflects the contemporary religious attitudes of the early
nineteenth century Protestantism.  On the voyage, Starbuck reflects these attitudes as
well and conflicts with Ahab who vows to fight the "inscrutable malice" of the whale and
break through the "pasteboard mask" of all visible objects.  That is, Ahab defies
conventional attitudes and fights against the Calvinistic sense of fate and "Innate
Depravity."  Ahab refuses to resign himself to the predestination of divine providence.
Melville, like his contemporary, Nathaniel Hawthorne, felt very much the "Puritanical
gloom" of his times, and as a Anti-Romantic, he also felt the dark forces of nature,
forces that lie at the bottom of the sea while the good, perhaps, is on the shore and in
the sky.