The packed room in which the Christ-figure lies is dark, save for a shaft of light which falls directly onto the statue, heightening the drama of the moment. Around the walls are benches; people sit in silent, awestruck contemplation. No one speaks; the reverence is tangible.

But if you think this is a scene in a church, think again. Few churches, after all, are packed at 2pm on an ordinary weekday, especially when there isn’t a service in progress. No: this is Britain’s premier art venue, London’s National Gallery, and though the atmosphere in its Sainsbury Wing basement might be reminiscent of a church during Holy Week, it is in fact the setting for an extraordinary art exhibition.

The Sacred Made Real was the brainchild of the gallery’s assistant curator of seventeenth- and eighteenth-century paintings, Xavier Bray. He was fascinated by Spain’s polychrome (or painted) wooden sculptures, examples of which adorn hundreds of Catholic churches the length and breadth of that country – but whose existence, as an art form certainly, is barely known beyond the Iberian borders.

Bray’s contention was that the sculptures had been horribly neglected by art history – their place, he realized, was pivotal in the development of the genius of great Spanish masters such as Diego Velazquez and Francisco de Zubaran. Bray persuaded the National Gallery to take the idea of an exhibition seriously – and then he headed to Spain for a three-month tour to find the most striking, and significant, pieces to include in his show.

In Spain Bray found no shortage of potential exhibits. ‘There was an extraordinary array of beautiful pieces,’ he says. He did, however, encounter what seemed like an insurmountable problem. ‘Most of the art works were owned by religious communities and priests – they were in ordinary churches, not in art galleries or museums – and the monks and nuns didn’t want to part with them. They were loved and revered by the communities and by the church congregations – they would be terribly missed if they were loaned. ‘About 98 per cent of the people I approached for loans simply shook their heads and said no,’ Bray remembers. ‘For a while, it looked as though the exhibition was going to be impossible.’

But like all good curators, Bray wasn’t going to give up easily. His trip to Spain had made him more convinced than ever that the country’s religious sculptures were not only a vital piece of artistic development, but also that they were truly living art that, though it emanated in a heavily religious Spain of 250 years ago, could and would speak to ordinary people in the secular UK of 2009. ‘I was really convinced of the relevance of this work,’ he says. ‘I simply knew that if I could only get these sculptures to London, they would have a huge impact.’

What made Bray so certain was that the sculptures seemed to him to have a twofold appeal. Firstly, there was the fact that the art world had never properly acknowledged them: while the work of artists like Velazquez is well known, historians had never really unpacked the fact that the models for some of their most stunning paintings were, in fact, the polychrome sculptures. ‘When you look at a painting like Velazquez’s Immaculate Conception, you are looking at a painting of a sculpture,’ explains Bray. ‘At the time, it was felt to be too presumptuous to use a person as a model for Christ or the Virgin Mary. Making a model and then painting the model symbolized the distance that there had to be between ordinary mortals and God – it made it possible for these great paintings to be painted.’

The sculptures, of course, were devotional objects in their own right, and were designed to capture the humanity, and the suffering, of Christ, his mother and his followers as possibly no art form ever before or since. They are extraordinary objects, painstakingly created, with intense attention to detail. The sculptors (whose names – Gregorio Fernandez, Pedro de Mena, Juan Martinez Montanes – are less well known than the great painters of the day, though the individuals were no less talented) went to enormous lengths to create as lifelike a model (often life-sized) as possible. Some of the sculptures have glass eyes to give an eerie reality: others have ivory teeth, human-hair eyelashes and bull-horn fingernails.

Confronted with these statues, Bray also realized that this discovery chimed with the zeitgeist of the art world – after all, what were contemporary sculptors like Ron Mueck doing if not trying, as the seventeenth-century Spanish sculptors were trying, to capture the physicality of human beings in a literal, 3D, immediate form?

But the problem still remained – how could Bray convince the Spanish priests, monks and nuns who owned these pieces of great art to loan them to Protestant England? Fortunately for Bray, there was a lucky break. Cardinal Murphy O’Connor, and other leading Catholic bishops in England and Wales, agreed to help oil the wheels with their counterparts in Spain to vouch for the wisdom of the loans. Bray went back to Spain to see if he could clinch the deals. ‘There were still some tricky moments,’ he says. ‘At one convent the Cistercian nuns voted by placing stones in a basket – black for no, white for yes. It took a long time before the basket was full of white stones!’ But what the nuns, and the other religious Bray encountered, could see was that he (though not himself a practising Catholic) was enormously reverential, and respectful, of the precious statues. ‘They knew they could trust me,’ he says.

And so the loan agreements were processed, and the art started to arrive. According to Bray there was some uncertainty at the National Gallery about how much drawing power an exhibition of hundreds-of-years-old religious art would have: but he always believed that, when the doors opened, the people would flock in. And time has proved him right: a fortnight into the exhibition, visitor figures are three times what was expected. On the day I visited, the gallery was a lot busier than it was for the last exhibition I visited in the same space – and that was on Picasso’s greatest works.

It’s clear from the demeanour of many of those who come that this is more than an ordinary trip to a gallery: many sit for a long time in silent contemplation, and the hush of the rooms is far more church-like than the normal gallery quiet. No lectures or tours are allowed: for the duration of the show, this is a place of thought and prayer as well as an exhibition space.

The visitors’ book is testament to how much the show has meant to many of those who have travelled to see it. ‘Almost too moving,’ says one comment. ‘Inspiring and thought-provoking,’ says another. ‘A marvellous spiritual experience for someone who is not a churchgoer,’ says a third.

Standing in front of not only that lifelike dead Christ, but other extraordinarily powerful pieces – de Mena’s Mary Magdalen, who is leaning over a crucifix, pain etched on her face, and the same sculptor’s St Francis in meditation – it’s easy to see why this show has touched so many. There’s an honesty and authenticity about the work: many of the sculptors and artists went to Mass, fasted and made retreats before embarking on a piece, in order to be as close to God as possible as they were working. That, coupled with the power of the fact that these are life-sized objects that confront us head-on with the humanity of Christ, the Virgin Mary and the saints, makes this an event that is far more than an art exhibition – as the crowds pouring through the doors of the National Gallery have testified.