Wise Blood by Flannery O'Connor.
"A Searching Novel of Sin and Redemption," this bizarre story from the queen of Southern Gothic literature is disturbing, hilarious, and profound. Returning war veteran Hazel Motes comes back to his Tennessee home to find it abandoned, precipitating a journey through a garish South, rife with the sacred and profane. Motes, an atheist, is (lack of belief notwithstanding) so preoccupied with theological concepts and symbols that God is inescapably and inexplicably behind every turning of his strange road.
O'Connor never makes interpretation easy, but the depth of her insight into depravity and redemption make this a powerful read.
Haroun and the Sea of Stories by Salman Rushdie
This phantasmagorical tale is part allegory/part meditation on the importance of story. Haroun, a young man from a city that has forgotten its name, travels across a bizarre landscape filled with riddles, puns (in two languages), and colorful characters as he seeks the source of the stories that can return hope to his land and family. It's a meditation on how narratives define us that will connect for storytellers of the pastoral kind.
Light reading, but with big images that will worm their way into your brain. You'll either love it or hate it.
Till We Have Faces by C.S. Lewis
It's a toss-up between this one and The Great Divorce, but this makes the list for its depth of feeling and excellent characters. Lewis's most critically acclaimed (and last) novel, Till We Have Faces reinterprets the ancient myth of Cupid and Psyche to poignantly illustrate the strangeness of the divine, the smallness of our souls, and the human urge to accuse God for our sorrows.
"When the time comes to you at which you will be forced at last to utter the speech which has lain at the center of your soul for years, which you have, all that time, idiot-like, been saying over and over, you'll not talk about the joy of words. I saw well why the gods do not speak to us openly, nor let us answer. Till that word can be dug out of us, why should they hear the babble that we think we mean? How can they meet us face to face till we have faces?"
The Scarlet Letter by Nathaniel Hawthorne
Completely underappreciated by the millions of high school seniors forced to read it, this American classic retains its power even 150 years after it was written. Central to the plot are themes of duplicity, sin, and redemption, and the question of what makes for true righteousness.
"No man, for any considerable period, can wear one face to himself and another to the multitude, without finally getting bewildered as to which may be the true."
The Power and the Glory by Graham Greene
This story of a washed up "whisky priest" during a period of Mexico's history when Catholicism was outlawed opens a window into the mind and soul of a man of God who is an utter failure (alcoholic, lecherous, cowardly), yet unable to forsake his calling, even though it may mean his martyrdom.
His ministry to a hopeless Mexico reeling from revolution intersects the lives of people at various stages of damnation and redemption, as he quietly journeys to his own personal Calvary.
Crime and Punishment by Fyodor Dostoyevsky
It's a classic, and it deserves the status. This is a murder mystery, but not of the kind we're used to. The mystery is not who committed the crime (the reader knows that even before the murderer does), but of what the murderer's crisis of conscience will prompt him to do.