Leonard Bernstein’s Mass is massive. And massively odd. I thought I had prepared myself for what was in store before this piece’s Proms debut on Monday, but actually I hadn’t. Maybe there isn’t really anything that can prepare you for the sight of several hundred Welsh singers, many of primary school age, gently boogying to the sounds of an orchestral frenzy, after singing a Latin translation of Psalm 130. And any work which rhymes “praying” with “Kyrie-ing” is always going to raise eyebrows, whether you know what’s going on or not.

With a (to quote the programme) “massed” children’s choir, a normal double choir, a choir of “Street People”, a rock band, a large orchestra, reams of pre-recorded tape, a treble soloist and a leading baritone called the “Celebrant”, it’s perhaps no wonder that this evening-long piece isn’t as frequently performed as other “masses” in the repertoire, and this performance was a seriously impressive achievement, logistically as well as musically. The choirs were all from Wales (several schools, the Cardiff-based youth choir Aelwyd y Waun Ddyfal, students past and present from the Royal Welsh College of Music and Drama, and the National Youth Choir of Wales and BBC National Chorus of Wales), as were the orchestral players (both the National Youth Orchestra of Wales and the BBC National Orchestra of Wales). Geographical variety was provided by the Rock and Blues Band, who had all worked with conductor Kristjan Järvi before, as well as by treble soloist Julius Foo, from Eton School, and by Danish Celebrant Morten Frank Larsen. Confusing, yes, but it all came together very well.

Not that the smoothness of the performance, however, detracted in any way from just how weird the piece is. It’s less a functional religious mass, and more a sort of musical theatre ritual work, abstractly dramatised in a naïvely expressionistic sort of way. Over the course of the piece, the Celebrant, “a young man of mysterious simplicity”, gains progressively greater influence over the Street People, but his simple message of faith gets increasingly diluted and misrepresented at the same time. He suffers something like a crisis of faith himself, and through a long, structurally vague section called “Things Get Broken” rediscovers peace of mind.

Bernstein’s music is famously diverse: the opening “Kyrie eleison” is complex and atonal but switches abruptly into the blues-inflected “Simple Song” through which the Celebrant first expresses his message. There is an obvious symbolism to this eschewing of the complex in favour of an easy, accessible compositional style, but it’s not as simple a story as that: many of the most intimate moments of the work, such as the orchestral interludes and the “Things Get Broken” monologue, return to a more “difficult” classical musical language. It’s through much of the more popular-styled music – a gospel section, solos for rock and blues singers – that the Celebrant’s message is most seriously diluted. It’s been called polystylistic, but it’s also conflicted.

Another upshot of the diversity of the work’s musical styles is that it makes for a seriously tough lead role. The Celebrant’s part requires a dramatic and vocal versatility which surely goes beyond that expected of most opera leads, and a heavy dose of charisma is needed to boot. I was only partially convinced by Morten Frank Larsen, whose interpretation was quite conventionally operatic – this meant that the lighter parts of his role (most crucially, the “Simple Song”) seemed rather laboured, even if he certainly had the vocal stamina and control to make a full evening of it. The Street People (ie. the RWCMD students and alumni) often had the same problem, seldom quite making the switch from operatic to musical/popular style. Ronald Samm’s charismatic cameo as a Preacher and an unnamed mezzo soloist with an excellently Americanized sound proved notable exceptions.

Anyone familiar with Bernstein’s own recording of the piece may have been disappointed by a comparative lack of swing in this performance; while the orchestra and band both sounded excellent (and BBC NOW’s orchestral interludes were highlights), it again all seemed a little straight. Maybe what’s most remarkable about the piece is that despite its ridiculous diversity, Bernstein never sounds insincere, no matter what style he’s writing in – so the rock-style sections actually have to be rock music, not just sound a bit like it, just as much as the more classical sections actually have to be classical. It’s a virtually impossible task, of course, but that’s what the work demands.

The choral forces all sounded very strong, filling the hall effectively and bopping along rhythmically as required. The schoolchildren were great, and so was young treble soloist Julius Foo. And Kristjan Järvi’s experience with the piece was very clear, especially as the evening progressed – there was never a hint of jadedness despite the scale of the undertaking. But while it was a hugely impressive effort from all concerned, this is a piece which needs the very strongest of performances to convince, and on this occasion a few doubts remain.