He doesn't present any actual argument against theism and most of the implied one is unpersuasive even to me, an unbeliever. But I do find that part of his conclusion resonates with me:
"Clearly, I saw now that belief in God, no matter how grounded, requires at some point a leap of faith. Either you have the gift of faith or you don't. It's not a choice. It can't be willed into existence. And there's no faking it if you're honest about the state of your soul."
I finished Arturo Pérez-Reverte's The Flanders Panel. He wrote the excellent Club Dumas, which was made into the not-so-excellent film, The Ninth Gate (In Johnny Depp's Pants*). The Flanders Panel is in a similar vein, with art conservation and chess taking the place of book-hunting and Satanism. I'm a far better book-hunter than I am an artist, and probably a better Satanist than I am a chess-player, so perhaps that's why I found the Flanders Panel disappointing. But really, I just think it's not nearly as good a piece of fiction. It certainly had its moments, but the resolution of the mystery was intensely unsatisfying.