A dying Christ reaches for help that never comes

Matthias Grunewald's 1516 Isenheim altarpiece reveals Christ's death without disguise - grotesque and terrifying - but it also…

Matthias Grunewald's 1516 Isenheim altarpiece reveals Christ's death without disguise - grotesque and terrifying - but it also features the most sensational resurrection, writes Jonathan Jones

IT IS ONE of the most terrifying and visceral works of art ever painted - a religious masterpiece that is harrowing to the core. The frightening reverse of the chocolate-box image of Old Master art and one of Europe's supreme cultural treasures, has its the permanent home in the local art collection of Colmar in Alsace.

The first thing you see, walking through the chilly Gothic chapel towards Matthias Grunewald's Isenheim altarpiece, painted between 1512 and 1516, is death. It is death without disguise, grotesque and terrifying. The wooden cross to which the dead man is nailed has a three-dimensional, trompe-l'oeil quality. Look at the wood: rough, cheap, ugly timbers have been crudely knocked together, as if the artist had asked a carpenter to make a mock-up. From this three-dimensional cross, the body of the dying man juts forward. His green fingers are twisted in horrible, grasping claws. This Christ is frightening, a dying man clutching at you, reaching desperately for help that never comes.

Christ dies in a terrible, empty desert place, with low massifs in the distance, and a lightless light, a depressing, dreary, empty darkness. His yellowish-grey, rotting flesh is covered with red sores, and the dead Christ's green corpse in the panel below is perforated and running with bodily fluids. It's horrible. Why did a medieval artist paint Christ in this shocking way? In its modern western definition, art has become inseparable from the idea of self-expression: artists make art to tell us what they think and believe. Religious art that was commissioned for churches and monasteries in the Middle Ages was not intended as anything of the sort. It had a social function. Art served to teach religious stories to the illiterate poor - it was the book of the illiterate. The Isenheim altarpiece was made for an Antonite monastery at Isenheim, near Colmar, that specialised in treating "St Anthony's fire", a sickness modern science knows as ergotism, caused by eating rye bread infected by a parasitic fungus. The horrific appearance of Christ's flesh on the altarpiece is not pure fantasy, but portrays symptoms the monks were trying to alleviate.

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This description is of the appearance of this altarpiece when it was fully closed. Originally a huge wooden structure with a series of hinged panels that could fold out to reveal different scenes, it was taken apart during the French Revolution and is now shown as separate paintings. There seems nothing more hopeless, more bereft, than the fate of its Christ - but if Matthias Grunewald painted the most dismally realistic image of Christ's death, his altarpiece also has the most sensational resurrection. Grunewald's Risen Christ floats up out of the tomb into a night sky illuminated by a vast sun, which shines around his brilliant white flesh. The cool nimbus of the solar disc, the red of Christ's stigmata, the gold of his beard, the gorgeously sculpted pale pink and blue of his robes - this painting's colours are some of the most brilliant in all art.

Grunewald's achievement came out of a great German art movement, a product of the German Renaissance culture. For, though it is now on French territory, the Isenheim altarpiece is a German painting to its marrow - German art's Sistine Chapel, even. Grunewald's amazing sense of colour allows him to transform the miserable darkness of his Crucifixion to the magical, illuminated night of his Resurrection - and to spellbind with coloured Gothic architecture, an intensely rich fantasy of angels, an exquisite mountain landscape.

You can see how this painting served the religious community of Isenheim and the sick they tried to treat. Yet there is a quality to it that is utterly personal, and lacks any power to reassure. Grunewald's faith is troubled. The religious experience his art records is a long, dark night of the soul.

In the most spectacular panel of all, Grunewald uses his chromatic gifts and free Gothic inventiveness to give horrible life to demons tormenting St Anthony in the wilderness. The white-bearded saint is pinned down by creatures that swarm from all sides. They have glassy, bulging eyes; one has the same sores and gangrene that disfigure Christ. Most eerie of all are the silhouettes on the skyline that emerge from a glowing, infernal ruin.

Grunewald's demons are more realistic - more concerned for us to believe in them - than the fantasies Bosch painted a decade or so earlier. This image seems more meticulous, as if it is carefully recording a confession. You are inclined to take it as autobiography.

In one panel, to the right of the Crucifixion, a figure of St Anthony stands wisely by - except that a devil is crawling in through a broken window behind him. It's another, even more confessional image of creeping madness: the terror at the window, the cracking of the glass by Something. And then there is the scariest scene of all, that same resurrection. Grunewald's Risen Christ is as frightening, as ghostly, as the negative image on the Turin Shroud. This Christ is not human after all, and his return from death is not consoling. It is a shattering fact that changes human history and sends the soldiers guarding the tomb falling over their armour, sprawling in fear.

As imagined by Matthias Grunewald, every aspect of the Christian faith makes your hair stand on end. He painted his altarpiece on the eve of the Reformation, and in it you see a man thinking about religious images with the same intensity Martin Luther brought to the Bible; there is something convulsive here, at the start of more than a century of religious war. Faith is not easy, in this vision: it is violent and extreme.

No wonder modern artists have been fascinated and inspired by Grunewald. Otto Dix and George Grosz explicitly echo the twisting fingers and diseased body of his Christ in their paintings of disfigured first World War veterans. The bird-monster who beats St Anthony with a stick inspired the bird-demons of the surrealist Max Ernst. Even Picasso painted a series of homages to it.

And yet the reason this masterpiece looks so modern is precisely that it is so deeply religious. Grunewald is not a Christian artist who sees beyond his time, but a spiritual thinker who meditates, in a way at once revolting, shocking and strangely beautiful, on the most terrible mysteries of his faith. In doing so, he creates a work of art that shakes you to your very being - even, or especially, if you believe in nothing.