The final word on divinity: Schoenberg never finished Moses und Aron. On the eve of a rare performance Bayan Northcott argues that he didn't need to

The evolution of Schoenberg's own faith was a complex matter, not to speak of its effects on his music

The challenge lies in the fantastic compression of his musical invention and the intensity of his expression

Arnold Schoenberg once claimed he was incapable of composing even ten bars without the aid of inspiration. Doubtless, the remark was intended to counter his fearsome reputation as a cerebral constructivist, but a survey of his output suggests it was essentially true. When the impulse was upon him he was able to work with an almost incredible certainty and speed.

The most famous instance was the composition, in a mere frenzied 17 days of 1909, of his expressionist monodrama, Erwartung: a half-hour score which innovates radically, and with an intricate finesse, on every level. But he evidently retained the power all the way from the post-Wagnerian surge of the early sextet, Verklarte Nacht, which took just three weeks of 1899, to the phantasmagoric late String Trio, set down in less than a month immediately after his near-death from heart failure in August 1946. The trouble with his reliance on masterly intuition was that if events conspired to interrupt a work in progress, he often found it difficult to pick up again - and the more ambitious, the more difficult. Not only is his legacy strewn with tantalising fragments, but three of his grandest projects remained unfinished at his death.

Significantly, they were all concerned with the search for religious belief. The earliest was a colossal theosophical oratorio entitled Jacob's Ladder - the more or less completed first part of which comprised an awesome build-up to a moment of transcendence. Then Schoenberg was recruited into the First World War. But he also seems to have developed a superstitious fear that he might be pushing into forbidden regions of the numinous: that if he ever completed the work, he would die. His third religious project - a sequence of Modern Psalms for which he furnished his own texts but only partially composed the first - was cut off, poignantly, by his actual death in 1951 soon after setting the words, 'And yet I pray . . .'

But then the evolution of Schoenberg's own faith was a complex matter - not to speak of its interaction with his philosophical and political views and with his artistic practice. Of his lower middle-class Jewish parents, his mother was a believer, his father a free-thinker. Schoenberg himself converted to Christianity at 24 - the one compromise in a fiercely principled life, some commentators have felt. Nevertheless, it was typical of his nonconformity to cleave, in predominantly Catholic Vienna, to Protestantism - which he soon supplemented by other more esoteric interests such as the Swedenborgian mysticism that seems to lie behind Jacob's Ladder. Then in the 1920s, just as he was laying down the laws of serialism, he found himself the object of anti-Semitic attacks, which evidently drove him back to religious first principles too. And when Hitler's minions duly drove him from Germany in 1933, his first act of defiance was formally to re-embrace the faith of his people in a Paris synagogue.

By then he had already composed as much of his second great religious project as he was ever going to: the unfinished but, arguably, complete music drama, Moses und Aron. The paradox turns on the realisation that Schoenberg's first two acts articulate a problem which could in no way be resolved, however many more he had added: that is, whether religious experience is communicable at all without betraying its very essence. On the most basic level, Schoenberg dramatises this through the conflict of speech and song. Moses, filled (in the opening scene with the Burning Bush) by the idea of the Infinite, Omnipresent and Unimaginable God which he is powerless to convey, can only declaim in stumbling speech patterns. Indeed, Schoenberg imagined him as 'utterly inhuman' like Michelangelo's great statue.

It has sometimes been suggested that Schoenberg dropped an 'a' from his other protagonist to avoid a title running to a fatalistic 13 letters, though the Lutheran Bible uses the shortened spelling too. Anyway, Aron, on whom it falls to transmit the Idea to the all-too-human Children of Israel, is cast by contrast as a lyric tenor - conjuring up the Unimaginable God in a stream of vivid imagery so that by the end of the first act, the initially reluctant Chosen People are ready to march through the desert to the Promised Land. But in the second act they begin to doubt once more a God they cannot see and, with Moses away on the Mount of Revelation, Aron reluctantly accedes to their demands. Schoenberg evokes the ensuing dance, orgy and collapse around the Golden Calf in a super-cinema scenario, but departs from the biblical narrative on Moses' return by having him smash the Tablets of the Law not at the sight of the idolatrous calf, but when Aron points out that they too are a mere image of the Idea. As the Chosen People resume their march, Moses sinks down in despair at his inability to express the Innermost. In his projected third act, Schoenberg planned to show him back on top: Idea triumphant over Image. But somehow in his remaining two decades he could never manage to compose it.

Brian McMaster's decision to open his first Edinburgh Festival as artistic director with a concert performance of Moses und Aron by Scottish forces under Richard Armstrong bodes bravely for the future, for the work is in no sense an easy option. Conventional prejudice might attribute its ardours for performers and listeners alike to the fact that it is by far the longest score Schoenberg composed in the dreaded 12-tone manner. Actually, Schoenberg thought of his 'method' - he never called it a system - as a means of restoring order to musical thought following the apparent dissolution of traditional tonality, and even, ultimately, as a possible step to the re-introduction of tonality itself. The real challenge, as in any major Schoenberg score, whether tonal, atonal, or 12-tone, lies rather in the fantastic compression and pace of his musical invention and the almost unremitting intensity of his expression. A tough subject demands a tough treatment. But there will doubtless always remain genuinely musical listeners who find the densely chromatic harmony and jaggedly nervous counterpoint, the continual cross- cutting of complex textures, the searing sounds of high strings, pungent brass and flailing percussion too much to take.

Yet when the work does reach the concert hall or stage, as it has done periodically since its first performance in 1954, three years after Schoenberg's death, it rarely fails to evoke a strong response - and not just out of the innate profundity of its dramatic content or its encompassing of the functions of both opera and oratorio. Ironically, from the Mosaic point of view, its impact largely depends upon a striking succession of musical images: the haunting overlay of sung chords by choral speech rhythms which symbolise the fire glowing through the branches of the Burning Bush; the strange intermezzo between the two acts with the People whispering their doubts in canon against fleeting flecks of orchestral colours; the extended Golden Calf sequence which packs a punch like a collision of Belshazzar's Feast with The Rite of Spring. And most memorable, the final moments of Schoenberg's score, with Moses' despairing cry of 'O word, thou word that I lack]', against a single rearing and plunging melody for the massed violins. Not the least suggestive of Schoenberg's endlessly pregnant and paradoxical achievement is that this ostensibly unfinished drama none the less culminates in one of the most moving endings in 20th-century music.

Usher Hall, Edinburgh, tomorrow at 8pm: box office 031-225 5756; relayed live on Radio 3