A.N. Wilson reviews Rowan William's book on Dostoevsky in the Times Literary Supplement.
The third paragraph contains some interesting insights on The Idiot, how knowledge of Russian Orthodox icons affects the understanding of Holbein's painting. I say interesting, but not essential: you don't need to know Orthodox iconography to grasp the emptiness of Holbein's painting or its use in the novel. It is, as Dostoevsky and Myshkin have said, enough to make one abandon faith.
Dostoevsky's novels are filled with characters driven by ideas, and many of these ideas are religious in nature. What endures for many readers are those stormy, passionate, conflicted characters, many of them wrestling with nihilism and religion. I know that being a Christian prone to intense doubts is one reason Dostoevsky's novels appeal to me, touching my psyche on a deep level.