Detail from ‘Dead Christ’ (1625-30) by Gregorio Fernández

In a global art world, cultural tourism rarely treads new ground; still more rarely do revelations about overlooked local corners illuminate swathes of the history of painting. But that is the achievement of the National Gallery’s scholarly, eccentric and intense new show The Sacred Made Real. Focusing on gruesome motifs of Christian suffering inspired by the Counter-Reformation in Spain, its themes extend way beyond religious art. Painting’s relationship to sculpture, the price of sensation, paths of realism, national traditions: its questions are pertinent, provocative and come alive through visual shock, delivered in images more violent and disturbing than in any contemporary exhibition.

Paintings by Velázquez, Zurbarán and Jusepe de Ribera here are superb but familiar; what disorientates in the Sainsbury Wing’s spare, black-walled rooms is their juxtaposition with polychrome wood sculptures produced during the same period for monasteries, cathedrals and private devotion, and until now never exhibited abroad.

Juan de Mesa’s macabre, life-size, swollen-eyed “Decapitated Head of St John the Baptist”, probably modelled on a criminal’s severed head, greets you as you enter. José de Mora’s disconcertingly larger-than-life “Virgin of Sorrows” in a luminous blue veil casts mournful shadows across a sepulchral gallery, as if in a chapel alcove. With such spectacularly emotional works, the National contends, the Spanish Golden Age painters were in constant debate.

‘The Immaculate Conception’ (1618-19) by Diego Velázquez

Thus Zurbarán’s “St Francis Standing in Ecstasy”, reinventing the patron saint of animals as an Inquisition philosopher, portrays the moment his uncorrupted body is discovered in his tomb: St Francis stands rigid like a statue in a niche, a stream of light outlining his body and casting its shadow against a wall. In an uncanny placement, this hangs above Pedro de Mena’s diminutive polychrome sculpture of the saint in the same cadaverously joyful pose; in a trompe l’oeil effect, his habit, painted as in the canvas with fine striations of brown to imitate a coarse wool weave, is held together with real cord.

Some of these polychrome sculptures are still used for processions and devotional purposes; all evoke black Spain, its primitivism, morbidity, superstition. Gregorio Fernández’s “Dead Christ” is a classical nude distorted by horror, his body thin and angular, bones pushing through flesh, lips and toes bluish grey, skin white as life drains away. Material details – coagulated blood, mimicked by cork tree bark painted red; glassy eyes; fingernails made from bulls’ horns – are used to layer on grotesque effects. De Mena’s “Christ as the Man of Sorrows” uses human hair, ivory teeth and skilful painting – purple bruising beneath the skin, rivulets of blood trickling down the body – to confront us with the physical presence of Christ. Clad in a black outfit shimmering like plastic, the towering, self-absorbed “St Ignatius Loyola” by Juan Martínez Montañés is the type to clear the street as he strides forward. De Mena’s dynamic “Mary Magdalene Meditating on the Crucifixion” steps towards us yet remains out of reach, rapt in anguish, clasping a crucifix, chestnut hair of twisted wicker coursing down like tears.

‘Virgin of the Immaculate Conception’ (c1620) by Juan Martínez Montañés

To modern eyes, this utter literalism looks like surrealism – Dalí comes to mind – or hyperrealism: eat your hearts out, Ron Mueck, Charles Ray, Maurizio Cattelan. How seductive these passionately expressive sculptures must have been to 17th-century congregations in a society with few visual images. Yet they had tremendous aesthetic as well as popular impact, for no works demonstrate more unequivocally the paradoxical Counter-Reformation demand that to stir the soul an artist must shock the senses – that extreme spirituality lay in extreme realism.

“Sometimes you might find a good painting lacking beauty and delicacy,” wrote Inquisition art censor Francisco Pacheco. “But if it possesses force and plastic power and seems like a solid object and life-like and deceives the eye as if it were coming out of the picture frame, the lack of them is forgiven.” That describes precisely Pacheco’s own “Christ on the Cross”, set against a black ground like a sculpture.

Pacheco taught Zurbarán and Velázquez, and was the latter’s father-in-law. “The Immaculate Conception”, painted when Velázquez was barely 20, shows the pupil’s attention to his master. The Virgin atop a transparent moon is at once naturalistic – a teenager with broad face, plump cheeks, long dark hair, modelled on Velázquez’s bride Juana Pacheco – and statuesque, her drapery piled in a mass of tangible folds like that in Montañés’s golden sculpture “The Virgin of the Immaculate Conception”, exhibited nearby. A year later, “The Venerable Mother Jerónima de La Fuente” is a masterpiece of psychological gravity and theatrical performance: Velázquez depicted the leathery, tough old nun, 66 years old when she set out to found a convent in the Philippines, as a Counter-Reformation soldier, brandishing her crucifix like a sword, fixing the viewer with an intimidating gaze – as if indeed she were marching out of the picture frame.

‘The Virgin of Sorrows’ (c1680-1700) by José de Mora

Illusion and reality, and plays between sculpture and painting are brilliantly dramatised throughout. In “Christ on the Cross”, the lifeless body emerging from impenetrable blackness became a wonder of Seville’s friary San Pablo el Real. “There is a crucifix which is shown behind a grille of the chapel (which has little light),” recorded Antonio Palomino, “and everyone who sees it and does not know believes it to be a sculpture.” Zurbarán’s signature, on a scrap of paper attached to the bottom of the cross, is an illusionary flourish: we question whether it is stuck to the wood, or to the picture itself. In “Saint Luke Contemplating the Crucifixion”, Zurbarán paints an artist painting a representation of Christ on the cross. In “Portrait of Juan Martínez Montañés”, Velázquez daringly depicts the sculptor at work on an unfinished bust of Philip IV. The king is identified by features delineated in bravura painterly outlines – a double portrait, therefore, showing both sculptor and painter in the act of creation.

“Not art, but life perpetuated,” Ortega y Gasset wrote of Velázquez’s portraits. Ruthless Counter-Reformation realism informed Spanish secular painting for centuries. When French artists grew sick of the classical beau ideal, they turned to Spain – perceived as so exotic and primitive that it barely belonged to Europe. Delacroix copied Goya, Manet and Degas met copying Velázquez at the Louvre. Degas’ “Little Dancer”, a bronze with the radical addition of muslin and silk, looks back to Spanish multimedia sculpture. A direct line links the Virgin of Sorrows to Picasso’s “Weeping Woman”. Crucially, Picasso’s reinvention of representational painting throughout the 20th century is rooted in Spain’s brutally realistic tradition.

In 1991, leaving a private viewing of “Las Meninas”, sculptor Juan Muñoz remarked “Now we go to lunch – but they stay! And that is the terror of Spanish painting.” The roots of that terror are here – excavated in this compelling show.

‘The Sacred Made Real: Spanish Painting and Sculpture 1600-1700’, National Gallery, London, to Jan 24; National Gallery of Art, Washington, February 28-May 31 2010