There is no shortage of TV shows about the supernatural—there’s even a show called “Supernatural”—but there are relatively few truly interested in religion. On mainstream television, “religious,” and especially “Christian,” is usually a euphemism for well-behaved and mildly inspirational, as in shows like “Touched by an Angel” or “7th Heaven.” But six episodes into its first season, “The Leftovers,” the HBO drama, has proven itself to be the exception: It is a show whose central concerns are not just religious but theological. It asks the question, what would have to happen for us to take religion seriously again? And would the world be better off if we did?

The premise of “The Leftovers” is deceptively high-concept. One October 14—we never learn the year, but the show is clearly set in our present—2 percent of the world’s population abruptly vanishes. Evangelical Christianity, of course, has a name for such a phenomenon: It is the Rapture, when the saved are taken bodily up to Heaven, while the rest of us are left to fight it out here below. But while the Rapture has inspired a significant body of films and books—most notably the Left Behind series by Tim LaHaye and Jerry Jenkins—the whole point of “The Leftovers” (based on a novel of the same name by Tom Perrotta) is that the Sudden Departure, as it is known, is not so theologically legible.

Indeed, one of the show’s central characters—a minister named Matt Jamison, brilliantly portrayed by Christopher Eccleston—has devoted his life to proving that the Departure is not the Rapture. He knows this because he has assembled a catalogue of all the crimes and sins committed by the Departed, which he insists on sharing with the world by handing out fliers on street corners. This mission naturally earns him a lot of hatred, including from his own sister, Nora Durst (Carrie Coon), who has the grim distinction of having lost her husband and her two children in the Departure. When Matt informs Nora that her husband had been carrying on an affair with their children’s preschool teacher, his callousness jars, but to Matt such truth-telling is a religious imperative. For if the Departed are not saints, then they have not been raptured, and the End Times are not upon us. As a Christian minister, he believes it is essential to interpret events through the correct theological lens.

But Matt is fighting a rearguard action, for in the world of “The Leftovers,” the most important spiritual fact is the decline of the mainstream churches and the rise of a variety of cults. (The Bureau of Alcohol, Tobacco and Firearms, we learn, has added “Cults” to its name and portfolio.) In the series’ most powerful episode so far, we see Reverend Matt desperately trying to save his usually-empty church from bankruptcy; despite what seems to be the assistance of providence (including at the roulette table, where Matt wins a huge sum), he ends up losing the church. To make matters worse, its new owners are the Guilty Remnant, the most important of the post-Departure cults to spring up.

All religions, "The Leftovers" recognizes, are founded on scandal and start life as an outrage.

The Guilty Remnant is a genuinely creepy group: They all wear white, take a vow of silence, and smoke continuously, as a gesture of self-destruction and a comment on the transience of all things. But the power of the show’s treatment comes from the deep respect it pays to this cult’s motivations. All religions, "The Leftovers" recognizes, are founded on scandal and start life as an outrage. What could be more scandalous than Joseph Smith finding the Book of Mormon on buried golden plates, or God appearing on Mount Sinai to dictate the Law, or Jesus coming back from the dead? All of these things defy common sense and the order of nature, which is why they cannot be accommodated in our old ways of thinking. They demand a change in the way we think about and respond to reality.