'Ready to rock?"

Those words may not be that unusual at Choral Arts Philadelphia's regular J.S. Bach rehearsals, but this week, their meaning is literal.

Artistic director Matthew Glandorf, who has been using the above words, is veering far from the basic choral repertoire with The Plague, an oratorio that singer/songwriter Andrew Lipke refashioned from his 2011 pop album of the same name.

The chorus and Lipke's own stratospheric vocals will share the musical foreground at the Bach@7 series Wednesday at St. Clement's Church. Among crossover-hardened listeners, you can almost hear the trepidation.

"We're still going to sound like Choral Arts - people who are influenced by singing J.S. Bach," said Glandorf, who originally proposed the project to Lipke. "And Andrew is going to sound like Andrew."

What nobody may be expecting are the opening moments. On a 1920s recording, an invisible fire-and-brimstone preacher, one Rev. J.M. Gates, proclaims, "He's coming! He's coming in judgment!" But if there's any apocalyptic finger-wagging in The Plague - a piece that examines holy wars, angelic conversations, and any number of end-of-times issues - it stops pretty much there.

Lipke himself - a charismatic, upbeat 37-year-old longtime Philadelphian who was born in South Africa - does not believe the end is near. "Not at all," he says pointedly. "I'm fascinated by matters of spiritual identity . . . how you're going to deal with an occurrence that might wipe out everything, and how that exists in your abject reality."

Glandorf's compass for The Plague isn't the choral/rock band juxtaposition of the Rolling Stones' "You Can't Always Get What You Want" but the more thoughtful melding of the later Beatles albums. "I've been thinking a lot about [Beatles producer] George Martin," Glandorf says, "and how naturally he made those recordings sound with studio trumpet players being themselves, the Beatles being themselves."

A key step in this project is taking The Plague out of the recording studio and translating the album into a predominantly acoustic medium for singers and instrumentalists the composer barely knows. "But in giving up control, you're also letting it have life," Lipke says. "I want to be with players who can bring their own energy to it . . . who bring their own soul into the music in ways that lift up the emotions. . . . And if you get good classical musicians . . . it's amazing."

It's also tricky - especially as so-called classical crossover hardly has an exalted history. Lipke knows he can't expect any chorus to follow in his vocal footsteps: Though never trained in voice, he has an upper range that many could achieve only with the help of helium gas.

Clearly, he's not some heavy-metal barbarian invading the classical world. Although he makes most of his living from playing in a Led Zeppelin tribute band, his solo activities include outreach concerts for the Philadelphia Orchestra and Chamber Orchestra of Philadelphia (such as a May 11 "Intersect" event at World Cafe Live) that he particularly relishes for the chance to play with string quartet. The solo albums that come out of his basement studio in Fishtown - he's made six, each the product of years of work - have high literary ambitions and lush, sophisticated arrangements. Before the current project with The Plague, Lipke took private counterpoint lessons with Glandorf.

Before arriving in Philadelphia at age 18 to study at the University of the Arts (with thoughts of becoming a formal composer), Lipke often performed in musical theater, such as Fiddler on the Roof, whether in Pretoria or as a teenager in Virginia. So his work with Choral Arts, he says, is not that big a leap - at least in comparison to his next commission, a piece for the vocal group Variant 6 that will be his first written for a group without himself as a performer. Recently, Lipke phoned Glandorf saying, "Dude! I think I've written a fugue! Can you check it out for me?" It turns out he had. Then again, when Glandorf suggested a change in his vocal writing for The Plague, Lipke decided to keep it as it was.

In some ways, Lipke's sensibility markedly diverges from that of the classical community. As a concept album, The Plague is hardly a pop version of a classical song cycle. Not bound by a classical step-by-step narrative, Lipke created something of a free-floating fantasy on his end-of-times theme, without an obligation even to express his own lyrics in any emotionally literal way. Sometimes he enjoys composing against his own lyrics, saying dark things in a manner that's anything but. A good example of the way the pop composer can mix tones and registers is the third track on the album The Plague, titled "Holy War": andrewlipke.bandcamp.com/track/holy-war.

To Glandorf, son of a Lutheran minister, the album is about achieving religious maturity. Living a spiritual life isn't about talking to angels, but having a daily dialogue with the temporal world. Lipke, son of a Methodist minister (he's not currently associated with a religious community), did not agree - but such varying interpretations are to be expected with concept albums.

Also, the album's songs can't help take on the color of their musical surroundings. The Wednesday concert's first half will be Bach's Cantata No. 19 "Es Erhub sich ein Streit," about Lucifer's fall from heaven - as sort of a bookend to Lipke's prerecorded minister predicting the Last Judgment.

How all of that shakes out with The Plague perhaps can't even be predicted at this point by anyone associated with the project. Some of Lipke's layered instrumental arrangements are now handled by wordless chorus. As much as Glandorf is determined that Choral Arts maintain its musical identity, he admonished his singers at a recent rehearsal for being "too polite."

And though one part of The Plague has Lipke singing "Hosanna" - more in the spirit of Leonard Cohen than in that of Bach's Mass in B Minor - some of the newly wrought vocal arrangements suggest a church choir.

Well, why not? It's in a church.