As I've mentioned earlier, the actual plot is derived from the Christian eschatology which includes the Rapture and the Tribulation, and both books use this is a device to move through the story, which adds an interesting element that, although I am not religious, is commonly known enough to be familiar, with a whole bunch of extras that steer the story away from Christian preaching and to a fantastic young adult read that also holds appeal to adult audiences, particularly fans of post-apocalyptic and dystopian fiction.