Yannick Nézet-Séguin stares down the daunting Bach Mass in B minor — ‘I’m not even scared.’

J.S. Bach seems to be pounding on the doors of heaven throughout his Mass in B minor, and with a magnitude that’s never been matched.

The infirm composer (1685—1750) was one year away from death, falling out of fashion, and going blind — his usually graceful handwriting deteriorating into something only his children could read — as he composed and compiled a mass that encompassed despair and celebration in unprecedented extremes.

Why, then, are the Philadelphia Orchestra’s upcoming performances under Yannick Nézet-Séguin (Dec. 5-7) the first since 1985?

Conductors alternately embrace and shrink from the piece. It wasn’t until roughly a century after Bach’s death that the entire piece was heard. The U.S. premiere came even later, in 1900 — just up the road from Philadelphia at the Bethlehem Bach Festival.

» READ MORE: Yannick Nézet-Séguin brings his other family, from Montreal, for its Philadelphia debut, with Joyce DiDonato

Nézet-Séguin performed the mass decades ago in Montreal when he was only 25, and maybe it wasn’t great. “I’m sure it was bad,” he said last week. “But Carlo Maria Giulini [his conducting mentor] waited many years because, he said, `I respected it so much that I forgot to love it.’

“Maybe I thrive on killing this intimidating weight," Nézet-Séguin said. "My credo is about love — love for the artist, love for the notes. Respect for an artwork will never prevent me from loving it and conducting it.”

Also, he said, “I’m not even scared.” But he’s not taking Bach casually.

Amid rehearsals for Alban Berg’s Wozzeck at the Metropolitan Opera (opening Dec. 27) and preparation for Strauss’ Die Frau ohne Schatten at Paris’ Theatre des Champs Elysees (Feb. 17), Nézet-Séguin has been marking the bowings on all of Bach’s string parts. And while Bach’s music is sometimes perceived as a precisely interlocking and well-oiled machine, this conductor believes that not all of the details are intended to fit smoothly. Expect some unlikely spice.

He quotes German choral conductor Hermann Max as saying, “We should think of baroque music like a baroque tapestry. There are so many details, and the variety of the details make the impression of an organic, harmonious whole entity.”

Just what that tapestry should feel like has been debated heatedly in recent decades — one element that keeps the mass from being performed. There’s no consensus on what it should sound like.

The late Arturo Toscanini adored the piece (“I wept the hottest tears of my life,” he exclaimed in a letter after studying the score) but only conducted the first section. Biographer Harvey Sachs believes Toscanini couldn’t decide how to handle the rest.

Wolfgang Sawallisch avoided it during his Philadelphia Orchestra tenure (1993-2003), telling recording engineer George Blood, "This work is such a monumental expression of the ability of the human mind to express the greatest thoughts in music, that I feel there is nothing I could bring to a performance which is not already on the page.”

He has a point. You almost have to perform the piece to hear all that is going on in it.

Much can be lost in the large blocks of choral sound that were popular in the post-Victorian era. Smaller is better. But how small?

Historic evidence suggests that Bach wasn’t writing for modern chorus but for a small collection of vocal soloists. How might that translate to modern ears? “The B-minor madrigal,” is how Bach specialist John Eliot Gardiner once referred to that approach.

Nézet-Séguin loves and respects the once-ridiculed, now-acclaimed Joshua Rifkin-led recording that uses only a single voice per part. ”But you know, it is a recording,” said Nézet-Séguin. “You have to have the right space for it.”

Even with a generous church acoustic, the Swiss-based Ensemble Corund took that approach on a 2000 tour stop in Bryn Mawr and reportedly couldn’t be heard very well.

For Nézet-Séguin, filling the Kimmel Center’s Verizon Hall means roughly 45 instrumentalists and 100 members of the Westminster Symphonic Choir. That’s pretty much consistent with recent concert-hall performances elsewhere.

In many ways, he said, he is building on the success of his similarly scaled Handel’s Messiah performances at the Kimmel Center. Many of this week’s soloists are longtime collaborators, including Carolyn Sampson, Karen Cargill, and Jonas Hacker, but with the significant addition of Benjamin Appl, who has made his name on art-song and concert repertoire.

Few singers are truly cut out for Bach, who must’ve had extraordinary voices in mind — assuming that he was thinking seriously about any performances at all. Bach’s relationship with audiences is something that can barely be imagined today: Writing for the church, he almost never heard applause. Positioned in a choir loft, he rarely looked his listeners in the face. Maybe public reception meant little to him.

The mystery deepens when considering why the piece exists. Or, really, what it is.

Bach spent his life in the service of the Lutheran church, composing brilliant cantatas on a weekly basis whether anybody appreciated them or not. Some of the works by which he’s most identified, such as the Brandenburg Concertos, went unheard in his own lifetime.

Disillusioned with life in Leipzig (and Bach, who did jail time for street brawling, was perhaps easily disillusioned), he wrote the opening movements of the Mass in B minor in 1733 to curry favor in Dresden. In 1749, Bach began expanding this curiously Catholic Latin mass with music recycled from what he had written before, maybe in the summing-up mind-set that prompted his late-period Art of the Fugue.

Certainly, the Mass in B minor seems to have everything in it: The 26 sections have soaring arias and duets, sparkling flute and violin obbligatos, and contrapuntal choruses with as many as eight vocal parts that continually top what came before.

Any amateur chorister has no doubt experienced panic-stricken moments when, at the mere turn of a page, you’re staring at a welter of notes that can be heard in your head but are too numerous to come out of your mouth.

Was ever-practical Bach just recycling previously existing music as a way of preserving his best material? Or was he, indeed, pounding on heaven’s door? The key of B minor even has poignance: It’s a mere half step from the wide-open elysian fields of C major. Like Moses, was Bach peering into the promised land with fears he’d never get there?

Cold scholarly evidence says no. The title, Mass in B minor, arrived long after Bach died. Some argue that the mass is four separate pieces — cantatas, perhaps. Most of it isn’t in B minor but in the related key of D major.

But Nézet-Séguin was willing to entertain that theory — along with esoteric ideas that are out there about numerology undercurrents in the piece.

“We find things in works that … if the composer were here, would say ‘No, I never intended this.’ That’s part of what makes them a genius,” he said.

“Many of them are not aware," he continued. "Maybe it wasn’t conscious, but at the end of the day they made this perfect work of art.”

davidpatrickstearns@yahoo.com

Bach’s Mass in B minor

Philadelphia Orchestra performances Dec. 5-7 at the Kimmel Center, 300 S. Broad St.

Tickets: $68-$195.

Information: 215-893-1999 or philorch.org.