El Niño, Barbican Hall, London

Good news - or even tidings of great joy - does not always travel as fast as it might. John Adams's Nativity oratorio for the new millennium, El Niño, has only now made it to London, rolling in alongside Previn's Streetcar for the Barbican's American Opera Week. El Niño is not, strictly speaking, an opera (whatever that is these days) or an oratorio (likewise). It is many things, or all things, to many people - a bit like attitudes to the Nativity story, which shift or change emphasis depending on religious affiliation.

So, let's "Sing of a Maiden" - Mary - and make her two people: an angelic biblical figure and an earthier, post-feminist Madonna; let's combine, in one performer, the role of Joseph with the voices of God and Herod; let's bring in a "Greek" chorus of fashionable countertenors and have dancers shadow their emotions; and, to top it all, let's simultaneously run a full-length film to chronicle the story in modern terms. In California, to boot. It has to be Peter Sellars, doesn't it? And it is. Once again (as with Nixon in China and The Death of Klinghoffer) composer and director are one - a creative partnership, from page to stage.

El Niño could only be American. In a sense, it is very much an offshoot of what used to be called the American Dream, restlessly doubling back and forth between the ancient and modern. The texts draw on many sources, from distant past to present day, with a heavy emphasis on the Hispanic. In American eyes, that makes it universal and politically very correct.

But it's effective. Orchestrally, Adams's score isn't much of a surprise, revelling from bar one in those pulsing, chugging ostinati whose vibrant rhythms have a way of creating light patterns in sound. To this he adds the primitive tinkling of tuned cowbells, reedy woodwinds and folksy strumming, be it from guitar or fiddles. But it's the voices - six soloists plus chorus - that give El Niño its musical identity.

Americans are generally good at word-setting. Adams is exceptional. Dawn Upshaw (Mary 1) has a Magnificat that "magnifies the Lord" in bright, ecstatic, wide-ranging intervals. It helps, of course, that this marvellous artist characteristically sings on the spirit of the text, pushing forward the words to carry the sound. Her climactic number - an impassioned setting of a lament by Rosario Castellanos for young people killed in the brutal overthrow of a Mexican youth revolt in 1968 - lends a 20th-century edge to the most powerful section of the piece, Herod's Slaughter of the Innocents, and how vividly she nails phrases such as "devourers of excrement", the word "excrementi" spat out repeatedly like a bad taste you can never be rid of. Adams undercuts this with another of his intricately rousing choruses, "In the Day of the Great Slaughter", even introducing a glint of humour with a Psycho moment for shrieking, stabbing violins.

The directness of El Niño relies a lot on a knowing naïveté. The film element underlines many such moments. When Willard White's voice of God declares he will "shake the Heavens", three LA cops in a car park fall to their knees at the sight of the guiding star, one weeping into his cheeseburger.

The performance was good. I shan't forget Mary 2 - the charismatic Kirsti Harms - or the rhythmically challenged BBC Symphony Orchestra under the composer's emphatic direction.