Until now, Isidore Isou has largely been forgotten or ignored by historians of culture. This is partly because he believed something which was absurd and impossible. He was a fanatic who believed that he was the Jewish Messiah sent to lead all humanity to redemption. If Isou is remembered at all these days, however, it is not as a would-be Messiah, but as the founder of an obscure avant-garde movement in Paris in the 1940s called lettrisme. Occasionally this movement has been cited as the missing link between dada, surrealism and situationism – but more often it has been dismissed by art historians as a curio from postwar Paris, a craze like so many other short-lived “isms” of the period.

The lettristes’ place in history has been further damaged by the fact that in his later years Isou acquired a reputation as a burned-out, megalomaniac madman. All of this may be about to change, however, in the light of a recently opened retrospective exhibition at the Pompidou Centre in Paris dedicated to Isou. The exhibition performs two important functions. Firstly, it reveals Isou to be a novelist, poet, film-maker and especially a painter of the first order. Secondly, it tells the story of Isou’s life. My own interest in this is that for the past three years I have been writing a book about the strange and oddly heroic life of Isidore Isou.

I met Isou only once, in April 1999, while researching a biography of the situationist thinker Guy Debord, who had at one time been Isou’s disciple but became his enemy – or, as Isou put it, “worse than a Nazi”. Isou was then 74 years old. He had been in bad health for a while and unable to walk for some months. He lived in Paris in two small rooms at the top of a building at 42 rue Saint-André-des-Arts. He had been there since 1965, never leaving the Left Bank except for a few trips to Israel. The legend was that he had paid off the mortgage by selling a sculpture given to him by his friend

Alberto Giacometti.

One of the reasons I wanted to speak to Isou was to help me better understand lettrisme and its place in the history of the French postwar avant-gardes. Although it is possible to discern a lettriste style and technique – the complex signs and symbols and sharp angular patterning of text and image – it is not always easy to be certain what they are for or what they mean. What Isou told me was that lettrisme was no less than the complete reinvention of culture and what it meant to be a human being. He was serious when he said this. Looking at the tiny bare apartment, and the visible poverty in which lived, I thought that he might be mad. Since then, having retraced his steps and pieced together his story, I have been sure about only thing; that nothing about Isou and his art is ever straightforward.

For one thing, Isidore Isou was not his real name. He was born Ion-Isidor Goldstein in the city of Botosani in north-east Romania. His mother affectionately called him Izu, the Romanian-Jewish form of his name that he precociously adopted as his avant-garde nom de guerre when he moved to Bucharest in his teens. He gives a vivid and intense account of this early period in his life in his first book L’Agrégation d’un nom et d’un messie (“The Making of a Name, a Messiah”) – a work which, published in 1947, when Isou was only 22, still astonishes today. It is a powerful and precise eyewitness account of the destruction of the Jewish world of Romania; a coming of age in a place that was once paradise but has now become hell on earth.

Isou is attacked and nearly killed by fascists during a pogrom, made to work in a forced labour camp, keeps trying to escape, is caught up in massacres, and all the time hears rumours of the terrible slaughter of Jews in the north of Romania.

Isou crosses the path of another young chronicler of the age, Mihail Sebastian, now praised by the likes of Philip Roth for his account of what it felt like to be a Jew in a city where everyone wants you dead. Sebastian became properly famous in the 1990s when his novels and private diaries were published in post-Ceausescu Romania. They became a sensation, revealing to a new generation the reality of the Romanian Holocaust, often dismissed by fascists and communists alike as a Jewish exaggeration. After the war, one well-known fascist belittled the persecution of the Jews in Romania as no more than “a few rags, a little money, a few working camps eight hours a day, with sleeping at home”. This was a widely held belief in Romania right up until the 1990s.

Isidore Isou knew the truth and wrote about it in L’Agrégation. Only the Zionists offered any real resistance, in his view, and he asked them to help him get to Israel. The plan was “fucked-up”, as he put it, when he got to the coast and there were no boats. Back in Bucharest, Isou hung out with his friends, visited brothels and got into fights, all the time hating the “Christians” who wanted to kill him. In a darkened cinema, he is about to make love to a “Christian” girl he has just picked up when he sees the first horrible images broadcast of the death camps in the east. He feels that he recognises himself in the piles of young corpses being swept into mass graves. “Don’t look at the Yids,” she says, pulling him back for another kiss. “They deserve it. They brought it on themselves.”

It was in this time and place, that Isou had the series of revelations that would become lettrisme. The first illumination came to him on a Bucharest street on 19 March 1942 when he was 17 years old. It was clear to him that the “old civilisation” he had grown up in was now crumbling away before his eyes.

He admired the surrealists because they’d had the courage to announce the “end of the Christian era”, a civilisation that he had come to believe was defined by mass murder and rape. He declared that he hated Christians, and unlike the weak-minded Zionists fleeing to Israel, he wanted revenge. He called for a “Judaism on the attack!” The question in Isou’s mind was the same question being asked then in the newly-born Israel: how to build the new world, how to found a new civilisation?

In 1945, Isou escaped from Bucharest, and after a hazardous and illegal journey across the wreckage of post-war Europe, he made it to Paris. Then, for a brief moment – he was only 20 years old – he was suddenly famous, courted by Jean Cocteau and André Gide, profiled in the New York Times and interviewed on film by Orson Welles. He was a star on the Left Bank of Paris, captivating all who met him with his charisma and good looks (he resembled a young Elvis). Isou quickly gathered a pack of well-read young rabble-rousers, including many Jews, as followers. This new gang of lettristes became notorious for their punch-ups, their weird, threatening poetry, their girls and their arrogance. Isou’s reputation was boosted by the fact that he was published by Gallimard (the rumour was the lettristes had threatened to beat up the distinguished owner of the house Gaston Gallimard, or at least firebomb his offices, if he didn’t publish Isou).

Throughout his life Isou’s creativity was unstoppable and this exhibition thoughtfully gives a clear sense of how vast and wide-ranging his output was. He wrote over 150 books, including works on medicine, psychiatry, religion, economics and history, as well as novels (he also wrote several erotic novels, one of which was banned and earned him a prison sentence

in France).

One of Isou’s most cherished ideas was the founding of a movement that he called “A Youth Uprising!”, arguing that in the post-war world the youth were the new proletariat. In 1949, posters appeared all over the Left Bank announcing that 12 million young people would soon be coming down into the street “to make the lettriste revolution”. When, almost 20 years later, during the events of May 1968, it seemed that young people really were making a revolution in the streets of Paris, Isou claimed the insurrection as his own; this was the moment, he believed, when Isou the Messiah would be truly in command of history.

What really happened during May ‘68, however, was that Isou had a mental breakdown. He stopped eating or sleeping. He talked incessantly to himself or whoever was nearby, babbling in different languages, some of which he had invented himself. Isou thought that he was invincible. To demonstrate this, in front of a small group of lettristes assembled in his apartment, he started smashing up mirrors and panes of glass with his bare hands until he was covered in blood. This so alarmed his lettriste comrades that they forced Isou into a car and drove him, still raving, to the psychiatric hospital of Sainte-Anne.

It was there Isou’s most tragic belief was revealed: which was that through the philosophy and practice of lettrisme he could find the secret of immortality. When, as he grew older, his body began to break down, the realisation that he would die like other humans destroyed his mental stability. Ignored by critics and diagnosed by psychiatrists, Isou saw himself as being trapped in Kafka’s Castle. He could see no way out and his pain intensified.

The great merit of the Pompidou exhibition is that it returns Isou to us not as a case study in delusion and psychosis but rather as an artist, and especially a great Jewish artist. The writing, drawing and paintings are all imbued with Jewish themes and symbolism. These can all be traced back to 1947 and the poem series called “Cris pour 5,000,000 de juifs égorgés” (“Cries for five million Jews with their throats cut”). This was a musical sound poem composed in Yiddish, German, Romanian and French and meant to be read out or chanted in the streets of Paris by a “lettriste choir” made up of young Jews, using drums and tambourines.

In 1950, Isou followed this up by publishing one of his most beautiful and important works – the Journaux des Dieux (“Journals of the Gods”). This “hypergraphic novel” consists of 50 plates of text, diagrams and drawings in a variety of colours, over-laying each other to form a fascinatingly cryptic puzzle. To see it close up on display here is like coming across a medieval illuminated manuscript – more to the point you can see at work the awakening of a creative, visionary intelligence that does indeed seem to convey some sort of religious significance.

Isou pursued Jewish motifs throughout his career. Above all, he returned again and again to the Hasidic tradition of Jewish mysticism in which he had been raised. He lived and breathed this culture as a child in Botosani, just before it was erased from that part of the world by the Holocaust. There is one singular creed in this tradition which Isou never lost sight of. This is a belief at the core of being an Ostjude, a Jew from the East, and it is contained in the formula that “man is the language of God”. You can find this belief at work in the paintings of Marc Chagall, as well as in the words of countless Rabbis and mystics from the Jewish Orient. From this point of view, for all its convoluted abstractions, Isou’s lettrisme suddenly comes into focus.

Isidore Isou’s last public appearance was in Paris, at the Amphithéâtre Guizot in the Sorbonne in 2000, when he shared a platform with the Holocaust survivor and Nobel laureate Elie Wiesel, a fellow Romanian Jew. Isou was quite ill by now and the medication was making him slur his words more than ever before. Still, he finished his talk on creativity to rapturous applause before being taken by ambulance back to his apartment.

Wiesel and Isou came from the same world. Like Wiesel, the sociologist Serge Moscovici (who had been Isou’s friend in war-time Bucharest) and many others, Isou was a member of a generation that had been obliged, through the experience of the Holocaust, to exchange Romanian identity for an imagined cultural and linguistic country that had Paris as its capital but no real or literal landscape. Isou, like all Eastern Jews of his generation, feared most of all being consumed by the murderous historical forces at work in his era. “Whatever I write,” he wrote on his arrival in Paris in 1945, “I write it because I am a Jew, and I am afraid.”

“This is why I have never written in Romanian,” Isou explained to me when I mistakenly put it to him, in April 1999, that he was in the tradition of Eminescu, the Romanian national poet whose real theme was his identity. “And this is because I am a Jew from a country which hates Jews.” It was not true, however, that he had not written in Romanian. He had begun his career contributing in his native language to the underground Zionist journal Palestina in wartime Bucharest. He wrote at the same time in his private journal – on display in the exhibition – that he would become “Mare Om din Lume” (“A Great Man in the World”).

Towards the end of his life, Isou never quite accepted that he had not achieved this. He was merely a prophet in exile, like so many prophets before him. This is why, during our conversation 20 years ago,

Isou said that although he now had French nationality, he was not and never would be a “French writer”. His ambitions, in keeping with his philosophical and religious principles, were universal. “I had to write in the language of mankind,” he said. “So, I chose French.”

For all his suffering Isidore Isou lived a long life. He died in 2007, at the age of 82. Throughout, he sacrificed everything – family, money, sometimes even his sanity – to the great project that was lettrisme. As the present exhibition demonstrates, right until the end, he lived and worked as a poet, an exile, a survivor and as a Jew.

The exhibition “Isidore Isou” is at the Galerie du Musée, Centre Pompidou, Paris, until 20 May.

Andrew Hussey is currently writing a book called Speaking East: The Strange and Enchanted Life of Isidore Isou