The Sixteen, Royal Festival Hall, London

Sometimes it behoves even card-carrying atheists to accept a little harmless mumbo-jumbo.

If you ask rational questions about what sort of "virgin" Mary was, and how the girl managed to get herself pregnant outside wedlock, you make it harder for the music written in her praise to cast its spell. Accept her as an archetype, however, and that spell works a dream – particularly with the wonderful music that Harry Christophers and The Sixteen have found for their Christmas programme.

They began with one of Arvo Part's German antiphons, all soft dissonances and echoing transparency, and followed it with Vaughan Williams's exquisite arrangement of the Herefordshire folk song "This Is the Truth Sent from Above", where the harmonies perfectly suit the pared-down grace of melody and text. For this, The Sixteen managed to sound like a real church choir, without a trace of bel canto in their whitened voices, but William Byrd's "O Magnum Mysterium", which came next, was a reminder of what virtuosi these singers are. Here, the QEH's dry acoustic, ideal for Part, made one wish for the multi-layered acoustic of a church.

Every work in this programme was a winner, impeccably delivered under Christophers' unfussy direction. I was rocked by Peter Warlock's setting of the "Corpus Christi Carol", whose constantly shifting chromaticism mirrored the queasy ambiguities of the text. I was charmed by the Basque carol "Gabriel's Message", and by Michael Praetorius's Lutheran hymn "Quem Pastores Laudavere". "Down in Yon Forest" – another English carol arranged by Vaughan Williams – interwove four sacred mythologies with strikingly surreal imagery.

The real splendours of the evening came with six- and eight-part motets by the 16th-century Spanish composer Tomas Luis de Victoria, in one of which each verse began in simple plainchant, before its polyphony unfurled with majestic richness. Yet paradoxically the best thing about this concert was the effect it had on its audience. After the final rumbustious, tambourine-slapping encore had died away, everyone trooped into the bar where, tanked up with mulled wine, an impromptu choir of hundreds sang yet more carols, in a four-part harmony almost as accurate as the one we'd had from the stage.