The first image visitors confront at the National Gallery's new Sacred Made Real show is an impressively realistic severed head, complete with detailed rendition of the anatomy of a neck.

Head of Saint John the Baptist, by Spanish master carver Juan de Mesa, was made in about 1625 and shows the lengths to which he and his peers went to make their subjects seem real.

Curator Xavier Bray believed the artist may well have used a real severed head as a model, as a lot of people were decapitated in those days.

Throughout the six rooms that make up the exhibition, there are painstakingly painted sculptures of saints, martyrs, the Virgin Mary and Christ, hanging on the cross and lying dead, naked apart from small loincloths.

They are interspersed with religious paintings by Spanish masters like Francisco de Zurbaran and Diego Velazquez, and the show explores the importance of one art form to the other in 17th century Spain.

While it is widely accepted that Caravaggio ushered in the powerful realism achieved by such painters, the National Gallery argues that religious sculpture was just as important and explained the sculptural effect of the canvases on display.

This is an exhibition of sculpture and painting, and the relationship between the two is very important, said National Gallery director Nicholas Penny at a press preview on Tuesday.

It is also an exhibition on a really neglected aspect of European art being displayed outside of Spain for the first time.

He said it had been fantastically difficult convincing Spanish churches to part with precious and often fragile sculptures, and the show, which runs from October 21-January 24, 2010, is the result of 10 years of work by curator Bray.

STIR THE FAITHFUL

The sculptures were artists' response to a challenge laid down by Dominican, Carthusian and Franciscan orders who wanted to bring the sacred to life, in order to shock the senses of the faithful and strengthen their devotion.

One painting by Francisco Ribalta shows Saint Bernard before a sculpture of Christ that appears to have metamorphosed into a living being, underlining the almost mystical union between religious followers and works of art.

In the same room hangs Zurbaran's striking Christ on the Cross, encased in an arched recess to recreate its original setting in a chapel attached to the sacristy of the Dominican friary of San Pablo in Seville.

Christ's lifeless body emerges from the blackness and achieves the illusion of three dimensions on two.

Nearby is Gregorio Fernandez's Ecce Homo, a sculpture of Christ as he was presented by Pontius Pilate to the Jews which features painstaking recreations of flesh wounds on his back.

In the next room, a model of the dead Christ is illuminated in an otherwise dark room for dramatic effect, blood still oozing from the wounds.

The close relationship between painters and sculptors in the Spanish Golden Age is underlined by the fact that in Seville, Francisco Pacheco, who painted the flesh tones and drapery on sculptures, taught Velazquez and became his father-in-law.

Early reviews of the show have praised the macabre images as disturbing and even erotic.

Painted or sculpted, these are real presences, wrote Adrian Searle in the Guardian. I left devastated and deeply moved.