Crucially, Ficino concluded: "The most important single factor about the world is that it is beautiful. Its beauty is an expression of God's beauty, for this world is His poem, His work of art." And human beings, as God's creations, are beautiful. Renaissance artists evince an unprecedented fascination with human anatomy - think of those acres of muscle-bound bodies painted by Michelangelo and many others - a fascination that often blends imperceptibly into science, as in the case of Leonardo da Vinci. Ficino's affirmation of the material world is symptomatic of the hugely optimistic, celebratory nature of religious expression in Renaissance art.
The growth of humanism was relevant to the emerging status of the artist, which in turn altered the relationship between artist and subject matter. From being an anonymous artisan employed to contribute to co-operative projects, as an artist you could be identified and associated with your own work, be a creative thinker with your own ideas and opinions. One of Albrecht Durer's self-portraits has provoked debate because of the way he seems to depict himself as Christ. There is no explicit evidence for this, but it is true that during his lifetime Durer developed a sophisticated, surprisingly modern vision of himself as an artist. And his self-portrait of AD 1500 seems symbolically to replace the divine presence with that of a secular humanist.
The greatest religious art of the Renaissance was produced when there were no gaps between spirit, intellect and artistic ability. Gaps began to appear, however. A little over a century after Raphael's Crucifixion, Peter Paul Rubens painted one of his most celebrated works, Le Coup de Lance, a dazzling, virtuoso depiction of a Roman soldier on horseback plunging a spear into the side of the dead Christ. The act of casual brutality occurs in the midst of a characteristically stormy composition, a veritable whirlpool of energy. Court painter to the Spanish Governors of the Netherlands, Rubens had spent time in Italy, and became the most brilliant exemplar of ideas current in post-Renaissance Italian painting. He was hugely energetic, capable and successful.
There is an unmistakable theatricality to his vision, already evident, though less pronounced, in his The Raising of the Cross and Descent from the Cross dated several years earlier. The nature of the artist's involvement with his subject matter has changed. To put it bluntly, Rubens is showing off. Rather than being subservient to a vision that is greater than himself, he takes the crucifixion as he would a mythological or court subject - as another opportunity to display his own expertise.
In Italy, Rubens had made copies of paintings by Caravaggio. A brilliant, temperamental painter, Caravaggio's ecclesiastical commissions were heavily criticised for their gritty realism, most notoriously when he reputedly used the corpse of a prostitute taken from the Tiber as a model for the dead Virgin. His pictures are set in a dark, shallow space, and inhabited by ordinary people - low-life friends, as often as not - playing the parts of Jesus, the apostles and the saints. The theatricality is overt. In other words, while Caravaggio set new standards of pictorial realism, deflating the wildly inflated excesses of Mannerist art, he also brings religious subject matter firmly down to earth.
If Caravaggio's sitters are ordinary people living the gospels, El Greco's are mystics in the grip of divine revelation. Though El Greco was one of the giants of Spanish painting, as his name makes clear, he wasn't Spanish. And he brought something of the Byzantine spirit with him to Iberia, together with the energetic distortions of Italian Mannerism. His religious compositions have a strange intensity, their figures transfigured, in ecstatic communion with an inner vision.
He was not typical of Spanish painting, as any comparison with the cool, brilliant realism of Velasquez illustrates. When he painted Christ on the cross, commissioned by a shady henchman of the Duke of Oliveras, who was in trouble with the Inquisition and was keen to make amends, he abstracted the suffering figure from its context and places it against a flat, dark background, as though it is a sculpture standing against a wall. But the understated virtuosity of the flesh-painting militates against any such illusion. In other Biblical paintings, Velasquez offers oblique views of the main subject, while foregrounding the activities of servants - this is symptomatic of artists' increasing involvement in the world around them.
As a measure of the changing significance of religious art, the example of 17th-century Holland is instructive. An incredible burgeoning of painting there was underwritten not by the traditional sources of patronage, the church, royalty and the aristocracy, but, in a taste of things to come, by the emergent bourgeoisie. While the church had been intimately associated with institutional authority since Constantine, the new art public was secular and materialistic. The essayist Zbigniew Herbert has memorably described 17th-century Holland, at the centre of a huge colonial empire, as "the kingdom of things".
In post-Enlightenment Europe, Christian themes never regained the centrality they had had for the previous thousand years or so. They remained as staples, but were not generally at the cutting edge of artistic endeavour. They appear in the sentimental story-telling of the Pre-Raphaelites, for example. Indeed, no less a person than Charles Dickens was moved to attack John Everett Millais's painting of Christ in the House of his Parents on the basis that it was blasphemous, which is surely as much a comment on Victorian mores as on Millais's picture, given the innocuous sweetness of the image. Christian ideas did filter through in the guise of nature mysticism. Casper David Friedrich's romantic landscapes were Christian allegories with a precise symbolic language - though it is possible to appreciate them without being at all aware of this.
The Victorian artist and writer John Ruskin, who rushed to Millais's defence, was typical of his time in being both a devout Christian and passionately interested in the natural sciences. His views accorded with William Paley's Natural Theology: the world that he observed was a reflection of God's ingenuity as a designer. But Ruskin was also uncomfortably aware that advances in geological knowledge literally threatened his world view - hence his remark about the tap-tap-tap of the geologists' hammers distracting him from his prayers. In desperation, he devised his own idiosyncratic taxonomy of the natural world, a world that, taken on its own terms, had failed him. His quandary, that the world was not enough in itself, without the presence of a guiding spirit behind it, has continued to dog each generation of artists since, to a greater or lesser extent.
After recovering from a serious illness in 1909, the German Expressionist painter Emil Nolde painted The Last Supper and several scenes from The Passion of Christ. "I obeyed an irresistible impulse to express deep spirituality and ardent religious feelings," he wrote, but he also worked with complete artistic freedom. As a result, his fierce, intense paintings are remote in kind from the decorative tameness of most religious art of our time. Nolde is usually mentioned in the same breath as the French painter Georges Roualt, the century's other major religious artist.
Roualt eschewed modernism and made convincing re-interpretations of classical religious themes. Trained in stained glass, nostalgic for the age of the Gothic cathedrals, and influenced by the Catholic writer Leon Bloy, he attracted ferocious criticism for his religiously charged paintings of prostitutes and pierrots, depicting the squalor of their lives with, it was claimed, a little too much relish.
In the absence of a central, living tradition of religious expression in visual art, religious artists have tended to be outsiders and visionaries. England produced several such artists, including William Blake, who lived in a strange inner world of his own, and Stanley Spencer, who painted Christ's story woven into the fabric of life in his native Cookham, and Eric Gill. Yet perhaps the most profoundly spiritual painter of the 20th century, Mark Rothko, moved away not only from religious iconography but from representation itself, into complete abstraction.
It could be that the amorphous luminescent clouds of colour in Rothko's paintings, tinged with mystery and eternity, provide a suitable screen onto which viewers can project their own spirituality. But he was certainly interested in providing an arena for spiritual experience rather than mere optical sensation. The brutal facts of his eventual suicidal despair are often taken as indicative of the bankruptcy of spiritual art in a secular age, or as the inevitable acknowledgement of the blank nihilism that underlies any bid to frame life's meaning in terms of existential humanism.
Against this background, Hughie O'Donoghue's vast pictorial cycle, The Passion, undertaken during the closing decades of the millennium, takes on unavoidable significance. O'Donoghue is not an oddball mystic, a Blakean visionary. His work lies firmly in the mainstream of the European tradition. His Passion is an attempt to address a spiritual theme through the medium of representational painting - and without the buffer of irony - in a secular age. It is perhaps too early to say whether he has succeeded or not. But it is clear that, in contrast to the hectoring tone of Byzantine art, which knows all the answers, his work reflects an honest uncertainty.
