SAN FRANCISCO -- "You speak to God in music," an angel sings to St. Francis of Assisi in Olivier Messiaen's consummate opera about the mystic monk. "He is going to answer you in music. ... Listen to this music that suspends life from the ladders of heaven; listen to the music of the invisible."

Audiences during the past few weeks heard luminous music -- of the visible and invisible, you might say -- as San Francisco Opera presented the first complete staging in the United States of the Saint Francois d'Assise, which premiered in Paris in 1983 and has been produced only three times in Europe since.

Messiaen's opera, eight years in the making, must be counted among the transcendent creations of the 20th century. It's an achievement not likely to be surpassed, in terms of scope or subject matter, anytime soon.

Experiencing it means a serious investment of time and patience, but I saw precious few deserters during last Sunday's matinee at the War Memorial Opera House; most of the packed house seemed to hang on every splendidly realized note, every arresting image.

And, from the roar afterward, the crowd clearly considered the five hours spent there (including two intermissions) thoroughly worthwhile. I certainly did. I'll never forget it, nor regret cashing in a whole bunch of airline miles to get out there.

Given the sheer magnitude of bringing Saint Francois to the stage, opportunities to hear it in person are sure to be rare. (The final San Francisco performance was last Thursday; those who missed it can hear the music recorded live during an excellent performance at the 1998 Salzburg Festival, released on the Deutsche Grammophon label.)

San Francisco Opera's new general director, Pamela Rosenberg, is responsible for bringing the work to the city named for St. Francis. It's a smashing calling card for her tenure. (The company's just-announced $7.7 million deficit probably means fewer such risks in the near future.)

Saint Francois marks San Francisco Opera's largest undertaking since its founding in 1923--- a chorus of 120, an orchestra of 97 (there wasn't any more room in the pit for 22 more strings Messiaen wanted).

Several instrumentalists had to be placed outside the pit -- the pivotal battery of xylophone and marimba players on one side, the three ondes martenot (an electronic instrument developed in 1928 that has a distinctive array of sounds and pitches) on the other.

All of this only begins to address the opera's requirements.

The title role, written for bass voice, calls for enormous stamina and sensitivity. Every other role in the three-act, eight-scene opera -- the angel, a leper, Francis' fellow monks -- makes its own vocal and mental demands.

The score, weighing in at 25 pounds, asks mighty powers of concentration and technical solidity of a conductor.

The stage director and scenic designer confront problems no less severe, given a work that defies conventional concepts of plot and momentum.

Normal expectations about music are thwarted. The composer's genuinely inimitable, virtually influence-free style follows its own rules of harmony, melody and rhythm.

Bracing dissonance; giddy, naive riffs; ravishing outbursts of lyricism -- all have equal footing here. Thick chords swirl through the orchestra in tight formations the Blue Angels would envy, punctuating the usually sedate vocal lines. Motives based on birdcalls, Messiaen's lifelong obsession, propel the music in complex ways. Part of the music springs from our world, our nature; part seems to come from another sphere.

But this still doesn't tell the whole story. More than anything else, Saint Francois forces performer and observer alike to confront the issues that occupied Messiaen all his life -- faith and devotion.

There has probably never been a more devoutly Catholic composer than Messiaen, who died in 1992. Many of his instrumental works capture his reflections on the divinity of Christ. He found in the life and works of the early-13th-century Francis the essence of his own beliefs; the ascetic's affinity for birds also struck an obvious chord.

The opera reminds me of Wagner's Parsifal, which, at its own snail-like pace, lasts roughly the same length and addresses similar concerns about faith and redemption, human and divine suffering, compassion, the search for truth.

Each work involves a life-changing kiss (Parsifal and the evil Kundry, Francis and the leper) and an unhealable wound (the one in Amfortas' side, the stigmata on Francis). Each ends in an affirmation of grace.

But where Wagner often strikes me as a man posing at devotion and toying with myth, Messiaen always sounds like a true believer. His faith is very specific, very real, very dear. It accepts no compromise but is never threatening or haughty.

You don't have to get on Messiaen's wavelength entirely or become a committed Catholic to receive what this opera has to offer. Like all great art that touches on sacred subjects, it rises above the specific.

This is nowhere more evident than in Act 1, when Francis faces a leper who despises the hypocritical sympathy he gets from the other monks.

Francis determines to kiss the leper, as the music stops cold. In San Francisco, that silence was elongated, creating the most awesome -- in the fullest sense of the word -- moment I have ever witnessed in an opera house. A deafening eternity of expectation.

I kept thinking during those terrible, tense seconds about all the unfortunate beings of the world that we better-off souls routinely fear and loathe and avoid. Everyone from the homeless to those living with AIDS seemed to be suddenly personified by that one leprous figure onstage, his deformed hands twitching as Francis oh-so-slowly approached.

For me, the opera's remaining acts did not always match the same combination of poetry and poignancy. The saint's "sermon to the birds" dragged; so did his death scene. But even when Messiaen's inspiration seems to slip a little, it's impossible to stop admiring the audacity and vision of this opera, the way he stretched himself and forces everyone else to stretch with him.

It was likewise impossible to stop admiring the performers in San Francisco. Willard White, in the title role, summoned remarkable expressive weight, even after some roughness crept into the voice. Chris Merritt made the leper a deeply riveting presence. Laura Aikin, the one-winged angel literally glued into a royal blue outfit (among costume designer Andrea Schmidt-Futterer's most inspired touches), was as wonderful in sound as in gesture.

Supporting singers, chorus and orchestra were all splendid. Enjoying a particular triumph was the company's music director, Donald Runnicles, whose obvious connection to the score animated everyone.

Between them, director Nicolas Brieger and set designer Hans Dieter Schaal provided many sights that will stay long in the memory, starting with the postmodern world where the action takes place, with its faceless figures in fedoras and trench coats shuffling up a curving path (or twisted cross).

Some images didn't come off persuasively, but lapses were few in a production that, ultimately, rose to the rarefied level of the opera itself.

Above all, the music and its messages hit home. When chorus and orchestra, after the saint's death, affirmed eternal life with a veritable blaze of good old-fashioned, uncomplicated C major, creating a soul-stirring thunder, you couldn't help but feel the uplift.

Here, at last, was the "perfect joy" that Francis always sought, the joy of "overcoming oneself" and bearing all manner of torment willingly, for the love of one's God.

Messiaen's first great composition was Quartet for the End of Time, a reflection on mortality and immortality. In a way, Saint Francois d'Assise may be the Opera for the End of Time -- the mystifying, revelatory, touching, effulgent product of a truly, uniquely joyous soul.