ART / Believe, or burn in hell for ever and ever: Andrew Graham-Dixon on the religious painting as horror show - as embodied in Sassetta's A Miracle of the Eucharist, on loan to the National Gallery

Religious paintings are usually denatured when they end up in museums, places that inevitably turn them into things which they were not originally intended to be. The museum tacitly transforms the religious painting into an object of aesthetic contemplation and art historical classification, rather than an injunction to the faithful. It becomes something to be admired for its formal qualities, to be retrospectively slotted into a school or a movement. It becomes a link in the chain of art history, too easily seen as an echo or a prophecy of other paintings, rather than a thing made to move, stun or astonish particular people in a particular place at a particular time.

Reading the catalogue to 'European paintings from the Bowes Museum', a sporadically wonderful exhibition of pictures currently on loan to the National Gallery from one of Britain's lesser known provincial art galleries, you are informed that exhibit number one, an early 15th-century predella painting of A Miracle of the Eucharist by Sassetta, demonstrates that 'his contribution to Sienese art lies principally in his convincing, though mathematically imperfect, construction of inner space' and in a 'unity of composition created by the skilful grouping of the figures'. This does less than justice to the ferocious piety of Sassetta's painting, its extraordinary marriage of righteousness and violence, and what must once have been the terrifying nature of its appeal to the lay Catholic imagination. This is a painting that is too fierce to be tamed, to be domesticated or secularised, and its message is unambiguous. Forget art history, forget composition and perspective and the skilful grouping of figures. Sassetta's picture is not really even a work of art, in the modern sense, it is a threat: believe, or burn in hell for ever.

A Miracle of the Eucharist is about the consequences of sinfulness, the perils of feigning faith, and the power of God. An unbeliever, clothed (probably symbolically) in black, has been found out in the act of receiving Communion. The Host, offered to him on a plate by the officiating priest, miraculously spurts blood and the unbeliever falls backwards, struck dead instantaneously, while a tiny black devil swoops down to snatch his soul as it emerges from his mouth. The priest looks down in amazement at the bloody wafer, the poleaxed communicant and the predatory winged devil reaching into his mouth. A group of thick-set Carmelite monks, heavy men with shaven heads and austere expressions, reel and fall to their knees in prayer, one of them uneasily supporting the dead man - with a gesture of succour that also looks like one of revulsion - as he topples. The queueing laity, waiting their turn to receive Communion, look at each other uneasily: amazed by the miracle, or perhaps newly daunted by the prospect of going, themselves, to the priest with open mouths. They have just become fully acquainted with their vengeful and savage God.

Sassetta's painting is a stern reproof to the very idea of placing such a work of art in a museum - a picture that makes you feel more than a little uneasy about looking at it somewhere other than in a church. Perhaps it is a great painting, in part at least, precisely because it continues so strenuously to resist categorisation as one. It makes that sort of aesthetic judgement seem thoroughly irrelevant - rather like admiring the dress sense of someone who has just sentenced you to death. A Miracle of the Eucharist implicitly damns anyone with the temerity to regard it as, merely, a painting, a quaint fiction. This really happened, is the message behind this carefully staged, meticulously created illusion (the picture commemorates the Miracle of Bolsena, said to have taken place in 1263). And if you are not careful, it will happen to you.

A Miracle of the Eucharist is designed to inspire fear and faith in equal measure. Every representational image creates its own world, but Sassetta's also gives the impression of taking you back to one. Modern Catholics are enjoined to believe in the miracle of transubstantiation as a literal truth, but the anthropophagous nature of the act of taking Communion is scarcely insisted on with Sassetta's fierce literalness, his absolute insistence on the connection between salvation and digestion. Eat God, his picture says (and really believe that it is the actual flesh and blood of Christ that you are swallowing), or get ready to puke your soul into blackness and hellfire.

Catholicism is often said to be a religion of ornament and artifice, whose ceremonies and churches are tainted with theatricality: one of the oldest traditions of Protestant thought links the theatre with the decadence of Popish Rome, and finds acting morally disreputable if not actually irreligious. But Sassetta's painting is, among other things, a diatribe against mere show, against ritual gone through without true faith. It is not enough to seem to believe: omniscient God will find you out; he will know, as you lower your doubting lips to receive him into your mouth, that your faith is not true. A Miracle of the Eucharist is a painting of an actor being found out in performance (the victim of God's rage is himself a Carmelite monk), an image designed to inspire all doubters with a positive terror of being discovered in their dissembling. God knows the true self, and it is naked and defenceless before him: as naked and defenceless as this human soul, a pitiful homunculus blindly stretching its arms out and emerging from the mouth of the stricken man like a newborn baby emerging from the womb. To die is to be reborn: another article of faith which the painting makes uncomfortably literal, while reminding those who look at it that their conduct here on earth will decide whether the midwives turn out to be devils or angels.

Perspective, in Sassetta's painting, performs as much of a rhetorical, persuasive purpose as it does a mimetic one. The space within which the artist contains his narrative is painted with supreme confidence, a lucid straightforwardness that makes the supernatural event taking place within seem doubly disturbing, emphatically other-worldly. Sassetta creates an entirely structured world, but it is also a world whose harmonies - those three, perfectly realised vistas through to architectural spaces beyond, the subordinatian of the entire design to the simple rhythms of arch and void - exist to be shockingly disrupted. The calm orderliness of Sassetta's painting, its lucid, confident creation of space, light, distance, give the appalling events of the miracle itself even more force: something terrible is happening, and it seems still more terrible and affecting because it is happening within this pictorial structure of perfect, measured spatial illusionism.

A Miracle of the Eucharist, seen in the National Gallery (or any modern museum), is a picture triply out of context. It is no longer in a church; the world and the people for whom it was produced have vanished into the past; and it has been cut from the far larger altarpiece by Sassetta of which it was once simply one element. At the last count, Sassetta's Altarpiece of the Eucharist was divided between three museums (British, Hungarian and Italian), the Vatican and a private collection. But this piece of it defies its displacement from its original setting by supplying, from within, an image of the world from which it has been torn. The picture's narrative is enacted in a church whose chapels conspicuously contain altarpieces (painted, by Sassetta, with a blurrily impressionistic touch that predicts Vermeer's paintings of images within images) like the one to which the painting itself once belonged.

The painted paintings in Sassetta's painted church - which may represent The Virgin and Child Enthroned and Christ in Majesty - also serve as reminders of the higher reality in which the sinful unbeliever has failed to believe. In Sassetta's world pictures are not, strictly speaking, art, but truth - and to consider them otherwise is a blasphemy that will be punished. The effect of all this is a form of sealing, an insulation of the picture from whatever modern setting it occupies, since it itself provides an image of its own original home, of the kind of place in which it once hung; and it also tells you precisely how it should be regarded, with unquestioning belief in the truths which it proclaims. It can never be entirely displaced from the world that brought it into being, because it carries that world, so whole and intact, inside itself.

(Photograph omitted)