A few years ago, I was caught up in a big research project about contemporary hymns (or “hymnody,” as they say in the trade). I listened to hundreds of hymns on Spotify; I interviewed a bunch of hymn experts. What, I asked them, was the most successful contemporary hymn—the modern successor to “Morning Has Broken” or “Amazing Grace”? Some cited recently written traditional church hymns; others mentioned songs by popular Christian musicians. But one scholar pointed in a different direction: “If you’re willing to construe the term ‘hymn’ liberally, then the most heard, most successful hymn of the last few decades could be ‘I Still Haven’t Found What I’m Looking For,’ by U2.”

Most people think of U2 as a wildly popular rock band. Actually, they’re a wildly popular, semi-secretly Christian rock band. In some ways, this seems obvious: a song on one recent album was called “Yahweh,” and where else would the streets have no name? But even critics and fans who say that they know about U2’s Christianity often underestimate how important it is to the band’s music, and to the U2 phenomenon. The result has been a divide that’s unusual in pop culture. While secular listeners tend to think of U2’s religiosity as preachy window dressing, religious listeners see faith as central to the band’s identity. To some people, Bono’s lyrics are treacly platitudes, verging on nonsense; to others, they’re thoughtful, searching, and profound meditations on faith.

Christianity Today regularly covers U2, not just as another Christian rock band but as one of special significance. In 2004, the magazine ran an article about Bono’s “thin ecclesiology”—his unwillingness to affiliate himself with a church—that sparked a debate about the health of organized religion. Rowan Williams, the Archbishop of Canterbury at the time, addressed the issue of Bono’s belief in a fascinating 2008 lecture about the place of organized faith in secular society. “Get Up Off Your Knees: Preaching the U2 Catalog” is one of several books exploring the theological ideas in Bono’s lyrics. Churches around the world have held “U2charists”—full services at which traditional church music is replaced with songs by U2. A few years ago, an Episcopal priest I know helped organize one at a church in New Jersey; the service, which featured a huge sound system, stage lighting, cocktails, and a bonfire, raised around forty thousand dollars for an orphanage in Cameroon.

Much of the confusion around U2’s faith stems from the fact that they’ve never been an “officially” Christian rock band. The ambiguity goes back to the band’s origins, in the Dublin of the late seventies, during the Troubles. In a country divided along sectarian lines, little about organized religion was attractive. U2 were teen-agers when they got together (Larry Mullen, Jr., the drummer, was just fourteen), but they were beginning to see outside of the faith traditions of their families. Bono’s father was a Catholic, his mother an Anglican. Adam Clayton (the bassist, English) and David Evans (the Edge, Welsh) came from Protestant backgrounds; Mullen had Irish-Catholic parents. In “North Side Story: U2 in Dublin, 1978-1983,” Niall Stokes, the editor of the Irish music magazine Hot Press, writes that the members of U2 were “primed” to ask what it meant to be Irish. They were “as close as you could get at the time, in an Ireland that was monocultural to an extraordinary degree, to a licorice all-sorts of nationalities and faiths.”

Their break with organized religion was probably inevitable. But it was still traumatic, which is perhaps why almost every U2 album contains a song about their decision to belong to a band rather than a church. (“One,” for example, is about the challenges of joining together with your friends to try and find God on your own.) Greg Garrett, an English professor at Baylor, a Baptist university in Waco, Texas, explains U2’s lack of religious identification in his book “We Get to Carry Each Other: The Gospel According to U2.” In high school, Bono, the Edge, and Mullen grew close to a faith community called Shalom, whose members Bono has described as living on the Dublin streets “like first-century Christians.” The group was a big presence in their lives during the recording of U2’s first two albums, “Boy” and “October” (“Gloria,” the best song on “October,” has a liturgical chorus, sung in Latin). The turning point came just as the “October” tour was set to begin: the Edge announced that he wanted to leave U2, because the twin demands of piety and rock stardom could not be reconciled. (“If God had something to say about this tour, he should have raised his hand a little earlier,” the band’s manager, Paul McGuinness, said.) Ultimately, of course, U2 stayed together: Bono, Mullen, and the Edge left Shalom. “I realized it was bullshit, that what these people were getting close to … was denial, rather than willful surrender,” Bono told an interviewer.

The tension in spiritual life—between discipline and vulnerability, order and openness, being willful and giving in—became U2’s central preoccupation, and gave it its aesthetic. During the Troubles, the band witnessed the consequences of an approach to faith that had become too organized and martial. Against that, they argued for “surrender,” in both its political and its religious senses. When Bono ran around onstage with a white flag during performances of “Sunday Bloody Sunday,” he was expressing not only an approach to politics but also an approach to faith (often, the song suggested, they were the same thing). U2 were learning to infuse their music with a sensibility that had been unreachable in their religious lives—a kind of militant surrendering.

As U2 grew up, they continued to talk about God without seeming to. On 1987’s “The Joshua Tree,” Bono combined sexiness with holiness, writing love songs sung to God, in the vein of the Song of Solomon. U2 has written a few straight-up love songs, like “All I Want Is You.” But, most of the time, when Bono uses the words “love,” “she,” “you,” or “baby”—which he does often—a listener can hear “God” instead.

Song lyrics are endlessly interpretable, of course—but, once you accept U2’s religiosity, previously opaque or anodyne songs turn out to be full of ideas and force. People sometimes sway to “With or Without You” at weddings, but the “you” isn’t a romantic partner (the line about seeing “the thorn twist in your side” should be a giveaway); the song is about how the intense demands of faith are both intolerable and invaluable (“I can’t live / With or without you”). “The Fly,” on “Achtung Baby,” seems a little overwrought as a love song, but as a song about the writing of the Gospels it’s surprisingly concrete (“Every artist is a cannibal, every poet is a thief, / All kill their inspiration and sing about their grief”). “Until the End of the World” is meaningless until you realize that it’s a love song for Jesus, sung by Judas, as portrayed by Bono. (This becomes especially obvious when the song is juxtaposed with scenes from “The Passion of the Christ.”) The best of these songs may be “Ultra Violet (Light My Way),” which sounds like it’s about a desperate romance, but is actually about the cruelty of God’s reticence: