Listening to Wednesday's world premiere of John Tavener's The Last Discourse at St Paul's Cathedral, performed by the cathedral choir under John Scott, reminded me of those terrible nightmares where mouthing even the simplest word is a tortuous effort. You certainly take note of the texts; in fact, you almost suffer them. Tavener describes this Discourse of Christ with the Disciples after the Last Supper as "one of the most disturbing and at the same time comforting meetings of Heaven and earth". And his method of conveying it in music is touched with genius.

He has his bass-voice Christ sing solemnly, painfully, sliding along the note in a style based on ancient Indian tradition, with an amplified double-bass for company. Stephen Richardson sang Christ, Barry Guy played double-bass (both performed from the first gallery) with soprano Patricia Rozario providing the voice of Holy Wisdom from the Whispering Gallery.

There's one particular passage near the end of the piece where Christ bids the comforter weep and rejoice, and the chorus responds, soft as gentle rain, with the words "Again I leave the world, Again go to the Father". It was a wonderful moment, exquisitely realised, and so different from the warning auguries that hover elsewhere in the piece. Tavener took his bows folded in a fur-collared coat, gesturing appreciatively to the choir like some bardic holy man of old.

The Last Discourse was a commission from Lord and Lady Marks and was the final item on a programme given in the company of the Prince of Wales and dignitaries from the Eastern Orthodox Church as part of "The Byzantine Festival in London 1998". Inevitably, memories of Tavener's contribution to Princess Diana's funeral service fuelled expectations, but The Last Discourse is very different to Song for Athene.

The rest of the programme was significantly varied. Dufay's opening motet Vassilisa, ergo gaude was shared between four members of the cathedral choir; there was plain chant, an organ hymn by Thomas Tallis and a Leroy Kyrie by Tavener's near namesake, the 16th-century English composer John Taverner. And yet even Dufay sounded relatively urbane next to three separate chant sequences from The Choir of the Syrian Patriarchate of Antioch.

The contrast between European mellifluousness and the raw, nasal but sensitively modulated singing of the Syrian choir suggested a Tardis trip across the centuries, from the burning commitment of Christ's immediate disciples to the received one of "traditional" believers. Or at least that's how Bairstow's Edwardian Blessed city, Heavenly Salem sounded, as English as a summer rose and no less lovely, though Jonathan Harvey's Come Holy Ghost was a good deal more involving, a sort of ochre aural mist, rising or falling with lines converging and texts shared between soloists and choir. So much colour, so much tonal imagination, though I rather fancy that Tavener's more austere essay made the stronger impression.