Those who have longed to hear the ceaselessly modern voices of The Crossing sing something old - as in J.S. Bach old - will finally get their wish, though in poetically fragmented form.

Having established itself as a new-music ensemble over the last decade, The Crossing and its cofounder/director Donald Nally now take on a massive work of compassion and suffering - political and social implications included. The project is built around the 1680 crucifixion oratorio Membra Jesu Nostri by Dieterich Buxtehude, interspersed with seven new companion pieces by contemporary composers from around the world, each with highly individualistic voices.

Collectively titled Seven Responses, the project unfurls over two discrete performances Friday and Saturday at the Philadelphia Episcopal Cathedral. The Crossing will perform the entire work in a marathon concert Aug. 21 at New York's Mostly Mozart Festival.

But the music is meant to do more than intersect with recent tragedies.

"Terrible things happen . . . like in Orlando, where you're not able to comprehend what happened and why," said Nally at a recent rehearsal of The Crossing. "But you're also looking at people all around having their own internal tragedies, some of which you know about, some of which you don't."

"The concept of somebody suffering for you," said German composer Hans Thomalla, one of the composers in Seven Responses, "is extremely contemporary." Example: Chinese sweat shops that make iPhones for the affluent.

"But human suffering," said Latvian composer Santa Ratniece, "hasn't changed throughout the centuries."

It is either quixotic or genius to base this huge project on the music of Buxtehude (1637-1707) and his text, by the medieval Belgian poet Arnulf of Leuven (who died around 1250). Known in English as The Limbs of Our Jesus, this Lutheran cycle of seven cantatas contemplates body parts of the suffering Christ - feet, knees, hands, sides, breast, heart, and face. Economical, almost ascetic but often as expressive as a Monteverdi opera, the piece has enjoyed a resurgence since the late 1980s, but isn't so famous that it would intimidate Buxtehude's modern counterparts.

The late Jeff Dinsmore, Crossing cofounder, had long bugged Nally to perform baroque music. "But I'm only going to do it if we can do it full out, with a baroque [instrumental] ensemble that really knows the rhetoric of baroque music," Nally said, "and put a contemporary ensemble right next to it so you can hear these color changes. ..."

Thus, Buxtehude's music will be performed by the Quicksilver Baroque orchestra. New pieces will feature the New York-based International Contemporary Ensemble (which created the opportunity to repeat the program at the Mostly Mozart Festival). The project cost $350,000 - a staggering amount for a contemporary-music chorus, but one generously seeded by the Pew Center for Arts & Heritage.

The contemporary composers - David T. Little, Pelle Gudmundsen-Holmgreen, Ratniece, Caroline Shaw, Lewis Spratlan, Thomalla, and Anna Thorvaldsdóttir - were selected partly for their literary acumen. Nally assigned a single body part to each; left to their own devices, most of them would have requested the heart.

The hugely diverse gallery of composers will ultimately meet in person at the concerts, though Ratniece believes they're already artistically united. "Somehow, we had to connect in our minds," Ratniece said in an email from Paris, "listening and studying the same music, reading the same lyrics, and thinking deeply about these big questions in life."

Nally (who was brought up Presbyterian) was unusually catholic about the texts the contemporaries chose. Pulitzer Prize-winner Shaw, in her piece, "To the Hands," has long lyrical wordless passages. Ratniece's starting point was St. Francis of Assisi ("I wanted a chance for the cello to sing the inner voice of Francis") but based her piece - titled "My Soul Will Sink Within Me" - on the correspondence between St. Clare of Assisi and St. Agnes of Prague. A slow, contemplative worker, Ratniece spent much of 2015 on her 15-minute piece that, if true to her form, promises harmonic saturation in the extreme.

Little, who recently had a successful Fort Worth, Texas, premiere of his opera JFK, was assigned the section on Christ's feet. "I had to do a lot of gruesome research on the practice of crucifixion and why it's so unbearable ... with the force of being held upward by the feet and hands, but also with the gravity pulling down," he said. Ultimately, he focused on the nails, which were said to have become amulets with healing power. Accompaniment of sorts comes from automobile brake drums hammered at some points; even more unorthodox effects arise by resting a battery-powered vibrator on the metal. His title: "Dress in magic amulets, dark, from My feet."

The heart went to Pelle Gudmundsen-Holmgreen, the 83-year-old Danish composer whose "Ad Cor" employs modern texts exploring the wounded heart, the joyful heart, and the cynical heart. "But in Pelle's last movement, everything comes together in this amazing architectural way," said Nally. "You should see the score. It's astonishing, these tiny little notes, because there are 24 voice parts and another 15 instruments."

Most radical, perhaps, is the composer who got Christ's sides: Thomalla, who now teaches at Northwestern University. His music is often built of complex, experimental collages. The texts he chose for his piece, "I Come Near You," are from the Song of Solomon and deal with religiosity in sensual terms - among deconstructed quotations from Buxtehude.

Thomalla speaks with greatest idealism about writing for choir. "A group of people singing together ... making harmony, making sense together in an extremely fragmented and polarized world ... I see this as utopian," he said. "Choruses are using their bodies to be part of a meaningful whole."

Among the Crossing singers, the body part likely to get the most important workout is the ears - which need to switch gears in ways never taught in conservatories. "Your ear works in different ways," said Crossing tenor James Reese. "In Baroque music, your listening is vertical. With new music, your ear has to travel around to different places."

Like America, Iceland, and Germany, just in Friday's concert alone. ...