I doubt I've read anything since The Brothers Karamazov that's caused me to consider my religious beliefs so deeply. Greene plumbs the psych of his protagonist, a nameless whiskey priest guilty of the mortal sin of fornication and countless venial ones, with an aptitude hard-earned through his own struggles to reconcile his Catholic faith with the world around him. The priest, the last one in Mexico during the Catholic purge, is compelled by powers greater than himself to continue offering confession and communion to those who request it, even though his life is in jeopardy and his own faith is nearly gone.
The book opens with the priest eschewing escape to offer confession to an old woman miles away who dies before he reaches her. It's a scenario that repeats itself throughout the narrative, as the priest passes up one opportunity of escape after another, bound to his calling even as it leads him closer and closer to his inevitable death.
And it is inevitable. From the first chapter onward, there's never really any doubt of the priest's fate. He lives in a world haunted by death and disgrace, the only honest priest left in all of Mexico. The only other priest significant in the narrative is Father Jose, spared because he renounced his Catholic vows and took a wife. The contrast between Father Jose and the whiskey priest is sharply drawn. Despite his safety, Father Jose is miserable and purposeless, spending what little screentime the story gives him bemoaning his uselessness and damnation, while the whiskey priest lives in fear of his life but has a purpose beyond simply staying alive.
The whiskey priest's penultimate scene finds him on death row, begging Father Jose to administer the host to him and thus save his soul, but Father Jose, despite receiving special permission from the government to do so, refuses. The book's most crushing moment arrives during the whiskey priest's last night, when he realizes that though he has absolved the sins of many, he cannot absolve his own and thus must die a damned man. It's a profoundly real and painful moment, a few lines communicating more about the human condition than most authors do in their entire career.
These few paragraphs can't do the book justice, and I strongly recommend that everyone here read it. I'm not Catholic, but I think it may be the greatest treatment of religion in English literature I've read.