Who was Olivier Messiaen? A pious, humble composer whose cosmic organ music can still scare the bejesus out of all those well-meaning, mulleted "teen pastors" and spiky-haired "youth ministry coordinators" strumming guitars to "Our Father" in churches today. From planting the seeds of avant-garde music for postwar composers like Pierre Boulez and Karlheinz Stockhausen to transcribing and integrating the complex tones and rhythms of birdsong (remember St. Francis of Assisi?) into his own music, Olivier Messiaen (1908-1992) was an experimental musician who did it all for the glory of God.

Although he is best known for the Gershwin-gone-gamelan Turangalîla Symphony and the desolate majesty of Quartet for the End of Time, the core of Messiaen's work is organ music. He treated the organ like a synthesizer (oh wait, the organ is a synthesizer, with pipes instead of circuits), using unusually bleak tones and murky textures to convey humanity's loneliness and longing for the divine. As you might guess from titles like Apparition of the Eternal Church, The Celestial Banquet, and The Glorious Body (Seven Visions of the Resurrected), the music encompasses many approaches to the sacred, from simple diatonic devotionals to archetypal, ambient blocs of sound to time-freezing voluntaries that herald the coming galactic apocalypse.

To commemorate the 10th anniversary of Messiaen's death, organist Paul Jacobs is touring the country, playing all of the composer's organ music in two five-hour marathon concerts. Since some pieces, like La Nativité du Seigneur, clock in at close to an hour, there will be several breaks during both concerts. Bring a discreet cushion for your behind. CHRISTOPHER DeLAURENTI

Paul Jacobs performs the complete organ music of Olivier Messiaen on Fri June 14 and Sat Jun 15 (St. James Cathedral, 804 Ninth Ave, 622-3559) at 7:45 pm, $15 or $25 for both nights.