Bach's St Matthew Passion, Glyndebourne Festival Opera, Glyndebourne

For all their innate theatricality, is there anything to be gained from staging Bach's passions? Especially at Glyndebourne, where "to drink from the cup of worldly sin" might be considered de rigueur and the indulgence of that lengthy supper interval could easily be misconstrued. The Last Supper? Hardly.

The point is that a masterpiece like Bach's St Matthew Passion is essentially a private meditation made public, and to elaborate its message, to lay metaphor upon metaphor, as Katie Mitchell's staging is wont to do, is – with the best will in the world – a distraction. It turns an act of communal mourning and contrition into something of a side show. Yes, the Passion of Christ is, whatever your beliefs, a compelling mystery play, but when it becomes a play within a play is there no danger of us becoming once removed from it? Cut off, as it were, behind the fourth wall? Are we not a more passive audience when not an active part of Bach's "congregation"?

Big questions – and Katie Mitchell is not without answers. Her central premise is a compelling one. We are asked to observe a community torn apart and brought together by the loss of its children. This isn't an invention on Mitchell's part: it is her somewhat literal response to lines in Matthew's gospel that speak of how we "forsaken children remain in Jesus' embrace". At the climax of the piece, with the line "unhappy Golgotha", Mitchell has the Mary figure, heavily pregnant, proclaim: "Innocence must die here."

We do not know how Mitchell's children have died; this could be another Beslan or Dunblaine. But as her congregation, her "community", enter Vicki Mortimer's familiarly shabby school hall, the children's images are projected on a makeshift screen. A shrine of tiny chairs and candles serves as a makeshift altar. For the purposes of the Passion, this is their church, a place in which to share collective grief.

And so begins the familiar story, played out in the cold half-light of fluorescent lighting while the snow sporadically falls outside. We are asked to accept that all the key characters are spontaneously and randomly drawn from this congregation – ordinary people drawn into extraordinary events – and that they are haltingly, falteringly, sight-reading their parts. And there's no doubt that such a contrivance lends a frisson of sorts to the idea that, for instance, Judas (Douglas Rice-Bowen) could have been any one of us. It makes the soprano's aria contemplating the corrupted child doubly poignant and lends further credence to Mitchell's basic premise.

Stage-managing this re-enactment of the Passion is the Evangelist, played with consummate clarity of utterance by Mark Padmore. His is one of those unfailingly true voices that all great storytellers should possess. Padmore is his own cast of thousands, projecting the text with that perfect balance between dispassionate objectivity and a startling immediacy. When he describes the sudden descent of darkness at "the ninth hour" his voice alone flicks the switch.

Writing about this production in the cold light of day makes one realise just how much quiet power it exerts. And yet there are distractions, moments when the Glyndebourne Chorus's accomplished but busy "method acting" precipitates messy ensemble, or where solo voices – particularly that of soprano Ingela Bohlin – virtually disappear into the ensemble. (In a contentious twist, Bohlin, the Mary Magdalene-in-waiting, becomes Jesus' bride in the opening few minutes.)

The most effective, the most moving moments are the stillest, where Mitchell has the good sense simply to let the music speak: Sarah Connolly's compassion for Peter in a gloriously understated "Erbarme Dich", or bass Christopher Purves' on-stage cello obbligato reflecting on the cross as a symbol of his own burden. The Orchestra of the Age of Enlightenment are a real presence throughout, Richard Egarr's direction conveying urgency and a searching integrity. But I still craved the removal ofthat fourth wall.

To 7 August (01273 813 813)