Sacred visions of godly gore

Tom Lubbock is awed and disturbed by the emotional intensity of Spanish sculpture and painting at the National Gallery

Oof! Actual size, accurately detailed, a severed head is displayed, lying on its side. The skin is wan. The dead eyes are half-open. The cleanly sliced windpipe and gullet are in plain view. This is a work in wood, The Head of St John the Baptist, by the Spanish master carver, Juan de Mesa. It's in the first room. It invites you in, as if into a chamber of horrors.

"The Sacred Made Real: Spanish Painting and Sculpture 1600-1700" is the National Gallery's autumn exhibition. The title suggests something encyclopaedic. In fact it's quite small. But its contents at times are overwhelming. The painters include greats: Diego Velazquez, Francisco Zurbaran, Jusepe Ribera. The sculptors are much lesser known - Juan de Mesa, Juan Martinez Montanes, Gregorio Fernandez - but they too can leave you gaping. For sheer intensity, for direct emotional power, I can't remember a show to equal it.

Its purpose is to demonstrate how these two arts had a common goal. Super-realism is what we'd call it now. The famous painters, the obscurer sculptors, were dedicated to creating a vivid, solid human presence (and more than human). They lived in a terrifyingly passionate age of faith. Their subjects were Jesus, the Virgin Mary, Mary Magdalene, St Francis etc. They strove to bring them, as if they were alive, to believers' eyes. And believer or not, the effect still works.

The show can be seen where the National Gallery usually puts its temporary exhibitions, in the basement of the Sainsbury Wing - and usually this windowless space makes a dull environment. On this occasion it's very effective. Darkness is what these images need. Whether paintings or painted sculptures, their bodies get real by materialising out of gloom.

With most of them, there's a moment, or more than a moment, when you take the image for something real. There's a shock of perfect illusion. With the polychrome statues, a lot of work goes into the colouring of the skin, the gleam of the eyes. Montanes' St Ignatius Loyola, all penitent, has glass tears applied to his cheeks. When you move past him, they glint, as if in motion, as if actually running. Meanwhile the painters give their attention to a well-shaded solidity. Zurbaran models the heavy white robes of the Carthusian order in tangible rolls. He presents St Francis in Ecstasy, standing bolt upright, emerging out of darkness into light. You could almost touch him? We’ll come onto that.

But the shock of realism is compounded with another shock: violence. An incredible amount of blood gushes and runs through this art. An incredible amount of thought is given to representing physical damage. Look at Fernandez' Dead Christ. It's not just the usual wounds - crown of thorns, hands, feet, side, whipped back. Both knees are deeply scraped (from carrying the Cross?). And the discoloration of the skin declares expertise. He’s been in the morgues.

Even if you're accustomed to contemporary art and its horror shows, this is a troubling tendency. For these are of course religious works. You may not belong to the religion in question, but this blood and corpse cult can still give you a shudder. These suffering bodies make it very hard to distinguish pity from sadism. Dark Spanish Catholicism is the great monster against which the Protestant-Enlightenment side of the European mind takes its stand. We suspect that the Church's glorification of Jesus sacrifice in the Crucifixion is somehow connected to the Church's torture of heretics. There's something deeply nasty going on.

And there's another problem, more strictly artistic. The show stresses the common purpose between religious paintings and religious sculptures. But we art lovers may feel uncomfortable with the way this equates Velazquez's works, say, with what are, in effect, waxworks. Painted effigies certainly have their power. But it's not the power of art.

This difference can be felt very acutely in this show, especially in the may we respond to those figures that are in such terrible suffering. You look at them, and out of compassion you imagine touching them. This is hinted at in a painting here, Francisco Ribalta's Christ embracing Saint Bernard of Clairvaux. True, Christ is bending down from the Cross, and he is hugging the Saint, not the other way. But with these images, the possibility that we might touch is always there too.

And what happens then? With statues, because they are 3D images, they really can be touched. And immediately their spell will break. However realistic they might look, when you actually touch them, you know it is only a real bit of wood, not flesh. A body in a painting, on the other hand, exists in an imaginary world. It can’t be literally touched. Just for that reason you can enter into the picture’s imaginary space and imagine touching the body there.

The Spanish artists seem to have been aware of this difference themselves. You see it in the way the painters and the sculptors treat violence. The statue-makers know that it’s no good trying to invite our touch. The statues are drenched with blood. Christ after having been whipped, Christ hanging on the cross, Christ laid out dead – blood, blood, blood. And the viewer's is compelled to recoil in a flinch of horror, disgust, outrage, maybe pity, but whatever, you don't want to touch, hold, embrace, stroke the poor violated body. And again, however well flesh and blood have been imitated, you can't ignore that it's hard wood producing all this blood – a repellent idea.

But turn to the paintings: they simply don't put in so much gore. Zurbaran in his magnificent blazing Crucifixion does all that he can to minimise the blood that his subject makes compulsory. A strong light falls from stage right so that Christ’s side wound is almost lost in shade. (Compare Gregorio Fernandez’ Christ on the Cross: blood vomits from the side wound.) Or look at Velazquez’ Christ after the Flagellation contemplated by the Christian Soul. Christ's back is turned away from us. The little boy Soul must slightly crane round to see the wounds we can't. (Compare Fernandez' Ecce Homo: his back designed to be viewed dripping with blood.) The suffering is recognised without being relished.

So this super-realist art is not only devoted to making horrors vivid. It has another side of it, to do with imagination. The paintings, unlike the statues, invite you to acts of compassion. The bodies are made to look real so that you can imagine their feelings, and then in return imagine touching them in pity.

The climax of the show is Zurbaran's greatest work: St Serapion. Second only to Fra Angelico, Zurbaran is the least violent of Christian artists. In this painting, all the grisly pleasures of his contemporaries are redeemed. Serapion was a medieval British monk and martyr, horribly butchered by some moors. But Zurbaran’s image utterly non-violent.

It does everything to show helplessness while eschewing blood. The friar's white habit is spotlessly laundered. The man himself is unconscious, slumped, but not conspicuously harmed. He's held up by his arms, which seem to be tied to the sides of the picture, but a support as much as a restraint. He hangs gently in the folds of his robes. The painting displays him simply for our mercy. He's the most huggable figure in art.

The Sacred Made Real: Spanish Painting & Sculpture 1600-1700, 21 October – 24 January, National Gallery, London WC2 (0844 2091778; www.nationalgallery.org.uk)