The first link in this post is to the Wikipedia article on the novel. Although it isn't long, it does hit the important points. It also has several links to reviews of the work, and to a page of annotations by the author.
The reviews say, correctly, that Sanderson created some memorable characters in this, his first novel. The first is Raoden, prince of Arelon, a small country near a now ruined city, Elantris, which was once, both city and inhabitants, the wonder of the world. The Elantrians had healing powers, lived for a long time, and performed various wonders. As the novel begins, Elantris is decaying rapidly, the result of some sudden disaster. The Elantrians are zombie-like creatures, original inhabitants, or exiles from Arelon, sent there because they have suddenly taken on the zombie-like aspects of the other Elantrians. Raoden himself is taken by this transformation.
Sarene, princess of a nearby country, is the second character. As her part begins, she has decided to marry Raoden, although they have never met in person. (There is video-phone-like communication possible, using intelligent servant beings who can communicate with each other over long distances.) She arrives, to be told that Raoden is dead. (He is legally dead, and his father seems to have forgotten him -- typical treatment for those who undergo the transformation to being an Elantrian, and the exile.) However, she is still part of the royal family of Arelon, by law. She begins to adjust, and tries to help the common people of Arelon, and eventually even those of near-by Elantris.
I will deal primarily with the religious aspects of the book. In large part, it is a religious novel, although mostly for the third of the three main characters. Hrathen, a high-ranking priest of the Shu-Dereth religion, sent to "convert" Arelon, by whatever means necessary, struggles with his own faith. Does he really believe anymore? Is his religion merely a means of achieving political and military power? What should be done with those who are adherents of other religions? Should they be killed? There are two main religions, but others are mentioned, one involving human sacrifice.
Hrathen finally comes to this position:
Right after this, Hrathen dies. The final words in the book are these:
"No," Sarene said. "When you speak of this man, let it be known that he died in our defense. Let it be said that after all else, Hrathen, gyorn of Shu-Dereth, was not our enemy. He was our savior." (p. 487)
It could be argued that this is somewhat exaggerated. Even Sanderson qualifies "savior" with "after a manner." The work of Sarene, herself, and the intense effort by Raoden to learn the secret of the Aons, are perhaps even more the saviors of Arelon, and Elantris. (The Aons are rune-like symbols, which were traced in the air with the finger, or on any surface, or could be sculpted into a surface, and were the means through which the pre-transformation Elantrians could perform various marvelous acts, such as healing and instantly transporting themselves to distant locations. Raoden finally discovers the reason why they stopped working, which I won't give away. He is able to make them work by the end of the novel.)
In summary, a great fantasy novel, with fine characterization, and considerable attention to moral and religious questions. I would by no means call it a Christian novel, but it does take questions of belief, and right behavior, so seriously that they are the core of the book.
I have previously posted on another book by this author, and will post again on another of his works in the near future, God willing.
*I label this "fantasy" because it is not based on extrapolation of some scientific phenomenon into the future, but depends on magic, that is, ". . . the power to use supernatural forces to make impossible things happen, such as making people disappear or controlling events in nature." (From the Google definition.) Most of the most important fantasy literature is "sword and sorcery" fantasy.