This is, in fact, a rare combination. There are no professional architects�let alone musicians, artists, or novelists�in the ranks of the saints. No Mozart or Michelangelo. No Titian. There is only the Blessed Fra Angelico�but he was a friar who painted rather than a painter.

One of the great modernist architects of the twentieth century, Gaudi was a scruffy mystic whose most famous work is the awesome, unfinished church of the Holy Family in Barcelona.

Hence the excitement over the fast-track cause for the beatification�the first stage of the journey to being declared a saint�of one of the great modernist architects of the twentieth century, a scruffy mystic whose most famous work is the awesome, unfinished church of the Holy Family in Barcelona.

The idea was first mooted by an organisation officially unconnected to the Church, the Association for the Beatification of Antoni Gaudí. It was formed in 1992 by two architects, his biographer, a priest (who has since died) and a Japanese sculptor, Etsuro Sotoo, who converted to Catholicism while working on the Holy Family. The association began printing prayer cards for private devotion to Gaudí, which are now in ten languages. There are reports of miracle cures, and requests for divine favours through the architect's intercession�for the successful completion of an architecture degree, say, or, in the case of a Peruvian priest, for a new church roof.

One of the founders of the association is Josep Maria Tarragona, the 45-year-old author of one of the most respected books on Gaudí. As it turned out, "there was much more enthusiasm for the beatification in Rome than in Barcelona", he told me recently at Opus Dei's residence in north London. Before the archdiocese of Barcelona allowed the cause to open in the local Church, the Pope got wind of the plan. "This Gaudí�is it true he was a layman?" John Paul II asked a Barcelona bishop who was in Rome for an ad limina visit. As soon as he was back, the bishop called the association to say he would open the diocesan stage at once. Last year, just 11 years after the association began gathering evidence, a 1,000-page dossier was lodged with the Congregation for the Causes of the Saints in Rome.

In Barcelona, the opposition has focused not on Gaudí's spiritual credentials�which few dispute�but on the appropriateness of declaring him a saint. José Maria Subirachs, one of the sculptors working on the Sagrada Familia, and an agnostic, argued that Gaudí was a "universal artist" who would be reduced by the Church's attempt to canonise him. Gaudí, said Subirachs, was appreciated by people of different religions or none; he belongs to everybody�not just to the Church.

But recognising Gaudí's sanctity in no ways threatens the universal appeal of his genius, says Tarragona. Rather, it locates it: the universal values of the Gospel and of nature are embedded in the architect's work. "If you know that Gaudí was a Catholic and a mystic, then you have some interpretative keys for understanding his art and appreciating its significance," he tells me. "That doesn't mean a non-Christian cannot understand Gaudí. But you cannot separate the man and his work from his faith."

Opponents of the cause insist, however, that Gaudí's Catholicism is accidental to his genius�even a limitation on it. "They believe that if Gaudí had not been Catholic, he would have been even greater," laughs Tarragona.

Opponents of the cause insist that Gaudí�s Catholicism is accidental to his genius�even a limitation on it.

The cause for Gaudí's beatification is especially galling to that branch of Catalan nationalism with a secular vision of a free Catalonia. Because nobody doubts the greatness of Gaudí, the nationalists are forced to try to separate him from his deep Catholicism, just as, faced with the Sagrada Familia as the city's icon, they once attempted to turn it into a "temple of culture". Gaudí, a deeply Catholic artist at the heart of the modern Catalan project, "breaks the template" of secularism, says Tarragona. For a Pope struggling against the mentality of a Europe unconscious of its Christian origins, that makes a Blessed Gaudí doubly compelling.

Gaudí was recognised from the beginning as a genius, the extraordinariness of whose work lay in his attempt to harness the forms of nature. From a childhood spent contemplating nature's forms, he observed that the abstract geometry of human architecture was foreign to nature, which instead had forms that are fibrous�wood, bone, muscle�and in shapes formed by gravity. Forms in nature, he saw, serve a function. If an architect looked for that function in his work, he could arrive at beauty; whereas if he sought beauty, he would reach only art theory, or some abstraction. Gaudí's major works�La Pedrera, the Güell Park, and not least the Sagrada Familia�are initially upsetting, because they are more like nature than architecture. For the same reason they are both captivating and timeless. Everything in his work, Gaudí would later say, "comes from the Great Book of Nature"; his task was that of "collaboration with the Creator".

Gaudí never bothered with attending classes for his architecture degree, but his commissions spoke for themselves. He fast became the city's best architect�and its most expensive. In his twenties Gaudí was a wealthy dandy, dressing extravagantly and giving orders to his workmen without getting down from his horse-drawn carriage. Like Picasso and Dalí after him, Gaudí "was conscious of his brilliance", says Tarragona. "He knew that Western architecture had come so far, and he was way ahead of it."

This was a recipe for endless egotism and future burn-out. But he was knocked off course by the refusal of the beautiful and wealthy Pepita Moreu to marry him. Another woman to whom he proposed shortly afterwards broke off the engagement to become a nun.

Almost no one questions the sincerity or depth of Gaudí's conversion at the age of about 31 to a life of austerity and prayer. The turnaround was gradual�Tarragona describes it as a "nine-year process"�but was no less dramatic for that. Thereafter, he remained alone, caring for the two surviving members of his family�his father and his orphan niece�until they, too, died.

The idea of the suffering artist is strong in Gaudí, but not the suffering of alienation, but of a loving response to God.

The turning point came in 1883, when he took over the plans for the Sagrada Familia, which would become the grand synthesis of his life's work. (The church is still unfinished, but now that it is being visited by 2 million tourists a year paying eight euros each, finance is no longer a problem, and work is proceeding fast.) He had by then come into contact with two key figures in his conversion: the Bishop of Astorga, Juan Bautista Grau, and Enric d'Ossó, who was later declared a saint. Gaudí began to shed his wealth, and to adopt the life of a pauper and mystic. Every day for the rest of his life he read the Bible, attended Mass, said the Rosary and confessed under the guidance of his Oratorian spiritual director. He seldom travelled, wrote little (although he was a great talker), and read selectively: by his bedside was the Année Liturgique by the Abbot of Solesmes, Dom Guéranger.

His asceticism could be extreme: people recall him nibbling on pieces of bread, or eating lettuce leaves dipped in milk. One Lent, in 1894, he had to be ordered to eat by his spiritual director after a radical fast left him at death's door. But his asceticism was mostly directed at his work, which he approached with characteristic obsessiveness and self-sacrifice. "Life is love, and love is sacrifice", the architect used to say in one of his bons mots. "Sacrifice is the only really fruitful thing." The idea is embodied in his great church, which he conceived as an "expiation" for the sins of the world.

But it would be wrong to portray Gaudí�as some writers clumsily do�as a "hermit". For his doctorate in journalism Tarragona has collected all the Barcelona newspaper and journal articles�about 3,000�which featured Gaudí in his lifetime, an average of two a week in his supposedly "hermit" years. "He was talked of a lot and was a recognisable figure about town," says Tarragona. "If you look at all the newspapers of the period�Catholic, tradionalist, Republican, masonic, anticlerical�without exception, they speak of him with absolute reverence."

Gaudí was active, for example, in a Catholic group of artists, the Cercle Artístic de Sant Lluc, headed by Bishop Torras i Bages; he also belonged to the League of Our Lady of Montserrat, a Catholic nationalist group which advocated a pluralistic, democratic Catalonia (as opposed to the integrists or Carlists, who spurned both Madrid and Rome). His nationalist credentials are impeccable: Gaudí, who always spoke Catalan, was arrested under the dictatorship of Primo de Rivera (1922-31) for refusing to speak Castilian.

A craftsman by training, Gaudí was demanding of his workers, but also endlessly kind. In his lifetime the Sagrada Familia was some way from the city centre, in one of the city's poorest barrios, where he was loved and respected. In 1909, during the so-called "Tragic Week", when a third of the religious buildings of the city were torched by anticlerical mobs, the Sagrada Familia was spared. When the next spate of church-burning happened after his death in 1936, the tombs at the church were desecrated - except for Gaudí's.

There are reports of miracle cures, and requests for divine favours through the architect's intercession.

The architect's struggle against his natural inclinations�especially his bad temper�was titanic. "God has given me the grace to see things with absolute clarity at that moment," he told a friend. "I have to say things just as they are, without beating around the bush, and of course people are annoyed." But it is only asked of saints that they struggle, not necessarily that they succeed. "I am a fighter by nature," he confided to his spiritual director the day before his accident. "I have always fought, and I have always succeeded, except in one thing: in the struggle against my bad temper. This I have not been able to overcome."

He came to exhibit to a heroic degree, says Tarragona, the virtues, both theological�faith, hope, and charity�and cardinal: prudence, righteousness, strength and temperance. The Carmelite Sisters of St Joseph who used to clean his room were told by their founder, Mother Rosa Ojeda i Creus, that they should take their spiritual cues from Gaudí. "We who have taken a vow of poverty must learn much from him", she used to say.

Accustomed to the Romantic icon of the artist as tortured and alienated, people find it hard to grasp that Gaudí could be at the same time mercurial and deeply prayerful. Interestingly, the idea of the suffering artist is strong, too, in Gaudí, but it is not the suffering of alienation but of a loving response to God. He used to say that the joy of artistic creation was so immense that if the artist did not respond through fasting, suffering and poverty, he would be in some way descompensado, "over-compensated".

But even if his personal holiness is not in dispute, what role do his works play in the cause for his beatification? If the Sagrada Familia is evidence of Gaudí's holiness, why does not the Sistine Chapel make Michelangelo a saint?

Tarragona believes that while plenty of artists have been touched by divine inspiration in their work, few�"perhaps only Bach, but of course he wasn't Catholic"�have exhibited the intimate relationship with God that is proper to a saint. "Mozart could write the Coronation Mass without having a devout life", says Tarragona. "God gave him the grace, as of course he did with Michelangelo in the creation of the Pietà or the Sistine Chapel. But in Gaudí we get both divine inspiration and a personal life proper to a mystic."

For a Pope struggling against the mentality of a Europe unconscious of its Christian origins, that makes a Blessed Gaudí doubly compelling.

Gaudí, in short, was a lay professional who became holy in response to divine artistic inspiration�which is why part of his cause must be his work. Because, in the last years of his life, he had total creative freedom, his works are inseparable from his holiness, says Tarragona, in the same way that it would be impossible to consider the sanctity of John of the Cross, for example, in isolation from the Spiritual Canticle or the Dark Night of the Soul.

One of the signs of the congruence of his creativity and his holiness was that Gaudí's greatness increased with age, unlike other artists�Mozart, say�who either burned out or died young. Like Bach, Gaudí worked every day of every week. After the Sagrada Familia commission, his life became for the next 40 years ever more geared to God.

So much so, in fact, that when people pointed out that the church would never be finished in his lifetime, Gaudí would just shrug. "My client", he would smile, "is not in a hurry."