If Francesco de Zurbaran’s ‘Christ on the Cross’ were the only work in the National Gallery’s powerful exhibition of Spanish 17th century painting and sculpture you should still move heaven and earth to see it. The 9ft 6in-high masterpiece was painted in 1627 for a Dominican friary in Seville, where it hung in a shallow niche behind a grille, illuminated by natural light from two windows just to the right of the viewer’s field of vision. On loan from the Art Institute of Chicago, it has never been in this country before, and perfectly embodies the show’s theme and title ‘The Sacred Made Real’.

In it, the figure of Christ crucified emerges from a black background, the left hand side of His body cast in strong light, the right in deep shadow.

The strong chiaroscuro pushes the figure forward, to a plane that feels closer to us than either the cross or background, further reinforcing the illusion that it is not flat but three dimensional. At the National Gallery we can see that the dramatic contrasts of light are dark are painted, but a visitor coming upon the image in its original setting would have mistaken them for the effect of natural light pouring in through the windows on the right.

But then the hyper-clarity of every detail – Christ’s strongly modelled musculature, His grey-tinged face, the blue-grey flesh around His wounds, the way light picks out the folds of His white loin cloth – adds to the illusion that the figure has a back as well as a front. Even the crumpled paper with Zurbaran’s signature is painted in trompe l’oeil, leaving the viewer to wonder whether it is attached to the foot of the cross or pinned to the canvas.

That Zurbaran tried to make his picture look as though it had been hewn out of wood and then painted isn’t surprising: he started his career as a painter of statues, not canvases. ‘Christ on the Cross’ was his first commissioned work in Seville, one of the cities in Spain that strictly enforced the medieval system of dividing painters and sculptors into separate guilds.

As a member of the guild of carpenters, a sculptor was allowed to carve a wooden statue and to prepare it with gesso (a mixture of glue and gypsum used as a ground for oil painting) but not to paint it. That could only be done by a member of the guild of painters, who, like Zurbaran, had been specially trained and licensed to add colour (polychrome) to statuary. It is entirely possible that the monks chose Zurbaran because they couldn’t afford to pay for a statue, and so asked the young painter to make the picture look as sculptural as he could.

Next to Zurbaran’s picture hangs a monumental wooden crucifix carved by Zurbaran’s contemporary Juan Martinez Montanes and painted by an artist whose name we do not know. Once again, the aim of both sculptor and painter is to make the figure look as lifelike as possible. Montanes gives the voluminous swath of white drapery around Christ’s hips such mass and volume that I think we are meant to understand it as a hastily improvised loin cloth made by wrapping His winding sheet twice around his hips in a loose knot. The anonymous painter is so meticulous in rendering both texture and colour that even from a distance we can see the blood from the wound in His side has already begun to congeal. In such works the two mediums fuse and it is impossible to say which is more important, the carving or the painting.

Apart from royal and aristocratic portraiture, the imagery in most Spanish painting and sculpture during the Counter Reformation is religious. It is therefore not ‘art’ in the modern sense because it was made neither for visual delight or aesthetic contemplation but as an aid to devotion.

Stimulated by the ‘Spiritual Exercises’ of St Ignatius Loyola, the faithful sought in prayer to form mental images of Christ, the Virgin and Saints. The painter or sculptor’s aim was to make the figure so realistic that it is as though the person to whom the prayer is addressed is standing there before our very eyes.

And so, instead of the idealised face that Raphael might have given to the Virgin, Velazquez gives her the features of a real young woman. In Montanes masterpiece – a full length statue of St Bruno shown preaching in the white habit of the Carthusian order – the superbly carved face is clearly based on that of a real person, perhaps one of the monks shown kneeling under the protection of the Virgin’s blue mantle in Zurbaran’s canvas ‘The Virgin of Mercy of Las Cuevas’. Look at Alonso Cano’s head of ‘St John of God’, where the saint’s stubble is created by smudging the wet brush onto the gesso surface rather than painting it bristle by bristle, and by ‘extending ’ the carved locks of hair with paint so that they appear to trail down over the figure’s forehead, ears and neck.

When Montanes carved his full length statue of the Jesuit saint Ignatius Loyola, he worked from the saint’s death mask. Here, the layer of gesso is so thin that the refinement of the carving of the veins in the forehead and hands is visible even from a distance. Notice too how the painter, Francisco Pacheco, adds painted shadows to real shadows on the cheek bones in order to intensify the three dimensional effect. By the way, the saint’s cassock is made not of wood but of fabric stiffened with glue (size) and is a later, 19th century addition. For a lucid explanation of how these statues were put together, there is a didactic show mounted around Francesco Gijon’s 1675 full length statue of ‘St John of the Cross’ in gallery one, just by the shop.

The curator Xavier Bray argues convincingly that it is not possible to understand the paintings or Zurbaran or Velasquez without taking into account the role sculpture might have played in the way they are conceived and executed. For example, Velazquez’s familiar ‘Christ after the Flagellation contemplated by the Christian Soul’ from the National Gallery is placed next to Gregorio Fernandez’s ‘Ecce Homo’ a life size standing figure showing Christ after the Flagellation. As the artist intended, we see the statue first from the front, where the all but naked figure looks pathetic but not horrifying.

Only when we walk round it to examine the sculpture from the back do we recoil, for the anonymous painter depicted the scourged flesh by removing one layer of gesso from the sculpture’s surface to create a gouge which he then paints blood- red. In Velasquez’s painting, the child kneels before the vision of Christ in such a way that he can see what we can’t – the bloody marks of the whip on his back.

The artists in this show are basically divided into those like Pacheco and Montanes, who used wood and paint to achieve the realistic effects, and ones like Fernandez and Pedro de Mena, who added human hair, ivory teeth and glass eyes to their figures, giving them, to my eyes, a slightly kitsch feel, like waxworks in the London Dungeon. But then, throughout the history of art, the more explicitly realistic the statue or painting, the less it has been valued as a work of art.

What is so original about this show is Bray’s courage and persistence in hunting down works of art that fly in the face of neoclassical notions of good taste. For unless you can enter into the imaginations of those who first saw these works and accept the absolute reality for them of suffering or ecstasy they show, you will miss half their power. This isn’t just a good show – it is one of the best I’ve ever seen at The National Gallery. And the catalogue by Bray brings this extraordinary and little known school of art to life.