Only three rock bands had albums that sold more than one million last year — the Black Keys, Mumford & Sons and a hard-rock outfit from Wisconsin with far less name recognition: Skillet.
What makes Skillet unusual is not just that its sales numbers rival two of the biggest rock acts on the American charts, but also that this quartet is an unabashedly Christian band that has won over mainstream rock aficionados without alienating its religious fans.
"That is a little bit of a trick," said John Cooper, the 38-year-old frontman, bassist and songwriter. "I tend to write songs I believe in, that get my message across in the best way possible and leave it as nonthreatening as possible."
Last week Skillet's most recent release, "Rise," a concept album about a teenager coming to terms with a violent world, made its debut at No. 4 on the Billboard 200 chart and No. 1 on the Top Rock Albums chart. And this comes after the group's last album in 2009, "Awake," reached No. 2.
Skillet's success seems to signal a growing acceptance of Christian rock at a time when rock is generally declining in cultural importance. While it is not unusual for Christian acts to score an occasional hit on mainstream radio, Skillet has become a regular presence there, emerging as a leader among Christian bands that have crossover appeal, among them Thousand Foot Krutch, P.O.D., Flyleaf and Switchfoot.
Mr. Cooper composes squarely in the heavy-metal vein and sings in an angry yet melodic rasp, recalling 1990s Nu Metal bands. His lyrics touch on Christian themes — struggle, salvation, a relationship with a heroic savior — but seldom mention Jesus. He makes no open overtures to win converts.
One single from the new album, "Sick of It," has been getting heavy airplay on mainstream rock stations and has climbed Billboard's Active Rock airplay chart to No. 12. Another Skillet single, "American Noise," is steadily moving up the Christian Songs chart.
"Skillet's done a really good job of straddling that line, where they are pulling from both the Christians and the mainstream rock crowd," said Jeff Cage, program director of the Edge, a rock radio station in Little Rock, Ark. "Skillet is not a Christian band that is overly preachy, by any means. You'd have to dig into their lyrics to even find out they are a Christian band."
Though some detractors have complained that Skillet does not proselytize enough, Christian rock stations, like KVRK in Dallas, still consider the band to be one of their own. "They are one of our core bands, and they have been for many, many years," said the station's music director, Chris Goodwin. "The power of their music breaks down any kind of barrier people might have."
Zach Kelm, Skillet's manager, attributed the band's success to relentless touring that includes both mainstream rock festivals and Christian music events. This week it will appear on Conan O'Brien's show, play a mainstream festival in Quebec City supporting Guns 'n' Roses, then travel to Wisconsin for the Christian Lifest. Later in the year it will tour with mainstream acts like Shinedown and Nickelback.
But Skillet has also been canny about using viral marketing — online contests and events pegged to the release of singles and videos — to motivate fans, who call themselves Panheads, to spread word about the album. The band has four million followers on Facebook.
Rather than simply release a single to radio in advance of the album, the band rolled out four singles on iTunes between early April and mid-June and tied those releases to animated videos published online by rock magazines like Revolver and Guitar World.
"We felt, 'They have this fan base — let's give them a real taste of the album, not just an appetizer,' " said Julie Greenwald, chairwoman of Atlantic Records, the group's label.
So far, critical acclaim has eluded Skillet; it remains a band with a big and loyal following but a low profile in popular culture.
"They are not necessarily the critics' darling," Ms. Greenwald said. "When you are not the critics' darling, it's hard to get media attention, so we just go right to the consumers."
The band also invited fans to contribute images to a video for the song "Sick of It" by sending Instagram pictures of themselves holding signs spelling out what made them angry (hopelessness, peer pressure, vanity, ignorance, hate, depression, bullying, apathy). Then Skillet pieced together a music video using the images and released that on YouTube.
Despite several lineup changes, the band has had a steady climb since it was formed in Memphis and released its self-titled debut album in 1996. The core of the group is Mr. Cooper and his wife, Korey Cooper, who plays keyboards. The other current members are the drummer Jen Ledger and the lead guitarist Seth Morrison.
Mr. Cooper grew up in a Baptist family in Memphis, where he was forbidden to listen to rock music — "Drums were considered evil," he recalled — but became fascinated with metal after hearing Metallica and Motley Crue at school.
For years Skillet won fans only in Christian circles. Then, in 2006, its album "Comatose" spun off the single "Whispers in the Dark," which crossed over to rock radio. It has since had nine songs on Billboard's rock chart, including the hits "Monster" and "Awake and Alive."
Mr. Cooper said that "Rise," inspired partly by school shootings and terrorist bombings, is about "a typical American teenager coming into adulthood and faced with how tragic and horrifying the world can be," he said.
By purposely keeping his approach understated, Mr. Cooper finds that many secular-minded rock fans feel that the band's messages are as moving as those of darker, more violent groups. "I get a lot of people at our concerts who say Skillet and Slipknot are their favorite bands," he said.
Indeed, some of the songs on the album can be interpreted as about a couple facing adversity together, but also can be read as a teenager turning to God for comfort and spiritual salvation. A good example of this ambiguity is the chorus of the song "Salvation":
I feel you keeping me alive
You are my salvation
Touch you, taste you, feel you here
You are my salvation
"I like that it can be interpreted in different ways," Mr. Cooper said. "If it was very clearly about Jesus, they might not get what we are saying."