When unimaginable musical elements enjoy an unlikely convergence, performers often do what's necessary to ensure that it happens again.

That's why the Malvern sanctuary of St. Peter's Church in the Great Valley has suddenly become a meeting point for musicians representing chronological extremes - the newly created and the utterly ancient - for a week of recording sessions. The music, Vespers by Philadelphia composer Kile Smith, is a piece so ambitious and singular that it might not otherwise be heard with any frequency, if ever again.

Emerging from blazing heat and intractable expressway traffic, singers and instrumentalists trickled in on Monday afternoon, the hubbub of the trip giving way to the peace of the church's country campus, where sheep grazed in the nearby graveyard. Many musicians exercised the option to perform barefoot. Even rehearsal mistakes turned into happy accidents. "I came in too early," says wind player Joan Kimball. "But Kile said, 'That's perfect. Let's do it this way.' "

Nobody could have guessed seven months ago, when the piece was premiered in Chestnut Hill, that Vespers would warrant a recording, which is being made for the MMC label with $40,000 in foundation grants. Though a quiet, steady presence among local composers, Smith is best known as a radio host on WRTI-FM (90.1) and as curator of the Free Library's Fleisher Collection of music.

He is also the author of approximately 100 pieces, many of them choral, and it was that fact that inspired Philadelphia's Renaissance wind band, Piffaro, to take a chance on commissioning a 70-minute Vespers service from him. Yet even Piffaro cofounder Kimball was surprised when rehearsals began. Put simply, "our jaws just dropped," she said.

"He's really a composer who knows how to generate an emotional life out of a few tiny notes - and that's the only thing that matters in composition," says conductor Donald Nally, who ran Philadelphia's Choral Arts Society for years but never heard a note of Smith's music until Vespers was proposed to him for his handpicked virtuoso choir, The Crossing.

"Everything fell together. It was almost as if I couldn't mess it up," said the composer during a break to readjust microphones. "Everything that was supposed to happen in order for it to work happened before I knew it. It had nothing to do with me, in a way."

The commission was unique in Smith's experience, if only because both groups are among the best of their kind and have few limitations. Immediately, he wrote a dizzying psalm setting with 16 different voice parts. Were Smith writing for a church choir, he'd be declared insane - and Nally initially was taken aback. "You kind of want to say, 'Please don't write anything just because you can.' That's always a disaster," Nally says. "But that's not what he did. . . ."

The piece is hardly typical for either of the ensembles concerned. Piffaro is in the business of playing music by composers who can only be consulted via Ouija Board. The Crossing, which exists only during intensive, discrete times of the year when Nally is on break from chorus-master duties at Lyric Opera of Chicago, gives programs that mix a wide range of contemporary works, never ones featuring hour-long works by a single composer.

The idea began as Piffaro was itching to do something different and contemporary. The traditional Vespers service has always been so open-ended that it does not constrict a 21st-century composer, and Smith's adult life has been steeped in Lutheran tradition and hymns - exactly the milieu that Kimball says best suits Piffaro's instruments.

What resulted has harmonic kinship with modern Anglican choral music, as well as some of the mystery of medieval music. The words familiar and original seem equally applicable. At best, the music is so direct and uncalculated - an expression of religiosity as opposed to an advertisement for it - that its antecedents disappear into a highly personal portrayal of psalm texts.

"A lot of people think religious music should be slow and profound," said Smith, "but the chorales come right out of Renaissance dance music. There's a lot of vigor."

Once Vespers was begun, he says, the composing process was like "breathing." That might seem odd to those who view religious music composition as an act of transcending one's self, rather than self-expression. The fact that self-expression is essential to Smith's religious music might seem even more odd considering the outwardly conventional circumstances of his life.

He grew up in Pennsauken, where his chief musical claim to fame was being cast in the title role of Amahl and the Night Visitors just before his voice broke. His main compositional education was at Philadelphia Biblical University, which accepted him only on probation because, at that time, his musical skills were seen to be so limited. He went to work at the Fleisher Collection after graduation, and has been there 27 years. Between raising three daughters with his wife in Fox Chase, he was composer in residence for the now-defunct Jupiter Symphony in New York City.

Reviewing a cross section of these past works, there's plenty of accomplishment and capability. His Vespers, however, seems to tap into a part of his life he hasn't always wanted to talk about: mystical experiences that he had as early as age 8.

In an e-mail, he described the first of these: "I was lying in bed looking at the ceiling and wondering about God, how could any of it possibly be true? And yet, how could it not be true? How could everything be here just by itself? At that moment, I was enveloped in a cloud of warmth that welled out of my ribs. . . ."

Such sensations, hinted at in past works, seem much more in evidence in his Vespers. Yet the process of getting them on paper may be beyond the help of divine intervention. Though Smith considers the Vespers psalm texts some of the greatest literature ever written, setting them to music is about time and work.

"When I wrote this piece, I had a vision of everybody sitting in a big church, a wide open space, at night. The problem with this vision is that it's nothing if the audience can't hear it. It took weeks to get those first four bars written," he says.

The rest, he says, has little to do with sacred vs. secular, but with artistic fervor. "It takes a while to settle in and find the nuggets within the text that make a personal statement with you," he says. "There's got to be a way that it strikes you, makes sense to you and is important to you. If that's not happening, why bother being an artist?"