Thompson painstakingly lays out the case that Melville's writing, by White-Jacket, becomes laser-focused on a personal theology that passes the baton from the anti-Christian Romantic humanists to the pessimism of Schopenhauer. (Thompson also goes into the extensive history of "Satanic Schools" such as the Ophites but especially the "Renaissance Humanists" and direct, significant influences on Melville such as Pierre Bayle.) From this manner of reading, Ahab is supposed to be a sympathetic character, the way that Renaissance Humanists read Milton's Satan as an anti-hero.
Most enlightening is the careful analysis of the literary techniques that Melville uses to hide his "sinister" beliefs. There is a sort of "triple-talk" that is subtle and changes the meaning of entire passages, etc.
Some points are weak, and my understanding is that modern Melville scholars fact-check Thompson's biography on Melville, but the core of the thesis is solid. A full understanding of Melville starts here.