“I’ve been listening to Moses und Aron in the car,” a friend said to me after the dress rehearsal, “but it doesn’t exactly give you much to hum along to.” Even so, my friend was there at the first night, along with a full house at the Wales Millennium Centre. Despite her shocked references to “scenes of Sodom and Gomorrah” in the second act, there turned out to be scant Sodom, no Gomorrah, and precious little in terms of magical stage spectacle – no burning bush, no serpent, no golden calf, no pillar of flame. None of this mattered: the performance was riveting from beginning to end.

The set for both acts was a panelled courtroom or lecture theatre, complete with raked seating, a bench for the absent judges, and a projector. Bright light pouring in through the rear window symbolised the burning bush, in front of which a shambling, inarticulate Moses stumbled about waiting for God’s voice to make itself clear. John Tomlinson played Moses in a way that resembled the older Brahms – the man, not the music – with a prophet’s grey beard and a clerical suit, rather the worse for wear. Aron was not the advertised Rainer Trost, who was indisposed, but the sensational tenor Mark Le Brocq, who stood in at short notice on minimal rehearsal, but who gave a performance that was just as gripping as Tomlinson’s, with a vocal line that, by contrast with Moses’ stammering Sprechstimme, sounded almost like bel canto.

The opera is a theological argument, conducted often in abstract terms (clearly rendered in the indispensible surtitles by Sophie Rashbrook and Jacqueline Pischorn) debating the place of God’s law in the life of mankind, and how this is to be communicated to the people, whose spiritual well-being depends not only on obeying this law but understanding it in the first place. Central to the debate is the chorus, broken up both musically and theologically into factions as fiercely divided as those in Israel today. There are the elders who are frightened for their safety because Moses has killed an Egyptian guard. A young couple debate on Moses’ being chosen to lead the Israelites. The notion of one god being stronger than the Egyptians’ huge pantheon is put up for discussion. Meanwhile, Aron becomes angry at Moses’ inarticulacy, and begins his string of miracles. In this production, both the serpent and Moses’ hand (which Aron turns leprous and cures again) are represented by a bible, and it is the same bible that stands in for the tablets of stone.  Nonetheless, Le Brocq’s long-haired, broad-shouldered Aron is a snake-oil salesman who uses visual trickery rather than verbal dexterity to win the Israelites over.

The chorus were in what looked like their street clothes (no costume designer is credited) and had to make the most of their musical lines in order to stand out as individuals. The smaller roles were well cast, with Richard Wiegold as a portly, gruff-voiced priest of the old order. The first act ended with a visible sleight-of-hand worthy of Doctor Dulcamara in The Elixir of Love: Aron pours red dye into a glass of water, prophesying that the Nile will turn to blood. The people accept his promise of a land flowing with milk and honey.

In the second act, Moses is supposedly up the mountain on his forty-day sojourn in the wilderness. The people are restive and impatient, and Aron has to promise that Moses will eventually return with a clear, comprehensible code of law for them to follow. The rows of seats in the lecture room in the first act now serve as a flea-pit cinema. The Israelites sit entranced watching a film of the Golden Calf, yet another of Aron’s illusions. Licentiousness breaks out, but in name only, with little of the nakedness and debauchery promised in the synopsis and the cast list. Even the Naked Youth, played by Edmond Choo, only gets to take his shirt off. It was more like teenage snogging than a full-out orgy. The hollow symbol of the Golden Calf was reinforced by the staging, whereby all that the opera audience saw of the Calf was the flickering light from the film projector shining directly at them into the auditorium.

Meanwhile the scene provided the opportunity for the orchestra to come into its own. The Dance around the Golden Calf was played with intensity and passion by the WNO orchestra under Lothar Koenigs, whose enthusiasm and knowledge of the piece communicated itself to the players in a way that could have taught Aron a lesson in crowd-control. Moses, returning from the mountain with the tablets, is enraged at the people’s idolatry. Again, he and Aron argue the primacy of the word over the image: Aron believes that without an image, the people will not understand the word. The chorus, angry with Moses, sneak away through the cinema exits, leaving Moses to destroy the Tablets of the Law – rather pathetically ripping the pages out of yet another battered bible. Aron goes off to lead the people towards the Promised Land. Moses is left alone on the stage, God’s word now silent.

Even though – and perhaps because – this production was short on spectacle, the very sparseness of visual detail forced the audience to gain the utmost from the superb singing from the chorus and the principals, and the inspired playing from the orchestra. Maybe, fundamentally, Moses und Aron, even in its incomplete state, is an oratorio after all.