EXHIBITIONS / The sacrament of holy oils: Andrew Graham-Dixon responds to two small miracles, paintings by Robert Campin on show at the National Gallery

ART HAS ALWAYS been a form of redemption, a transfiguration of the commonplace. The very act of turning something into an image, whether it is a bison traced on the wall of a cave or a milk jug painted by Chardin, alters and aggrandises it. Even the simplest representational picture carries an occult charge. Art is transubstantiation, and the eucharistic ritual of the Mass, where wine becomes blood and bread flesh, has a lot in common with the primitive and ritualistic activity we call painting.

Painting and religion have always had a close and often uneasy relationship with one another. To be an artist is to arrogate for oneself the powers of divinity. Painting transforms the mundane, both metaphorically and materially. Painting is itself made out of ordinary things that have been transfigured, the base matter of earth or oil or ground-up stone. Every painter is a shaman.

Two tiny pictures attributed to Robert Campin, a painter who worked in Tournai in the southern Netherlands in the first half of the 15th century, are the subject of the latest display in the 'Brief Encounters' series at the National Gallery. One is from the museum's own permanent collection, one has been loaned by the Hermitage in St Petersburg, and they share a common theme: The Virgin and Child in an Interior. But what they both seem to be about, at a deeper than merely iconographic level, is the relationship between the everyday and the miraculous. Their true subject is the elevation of the mere, the making sacred of the real. It is the essence of the story they tell, and the essence of their nature as images. They propose a subtle congruence between the miracles of Christian legend and the miracle of art.

In each painting, an entirely ordinary event is taking place. A baby boy has been undressed by his mother. It is bath time. In the Hermitage painting, the mother warms her right hand against the fire because she does not want her touch to chill the baby. In the London painting, the brass basin containing the water in which the baby is to be bathed has been placed next to the fire. These small details, conveying unexceptional motherly solicitude and the frailty of any small child, serve two opposing ends. They enhance the utter ordinariness of the scene, but precisely because they do so they also emphasise its extraordinary nature. Ater all, this baby is not just any baby, but the son of God. The paintings make their point economically. He roughed it; God submitted to the ordinariness of life as we know it.

So recognition, the familiar mundanity of the image, is charged with a sense of shock, although that has been necessarily diminished by the passage of time. Campin's down-to-earth interiors have now acquired the alienness and exoticism of the distant past. What once must have seemed real now seems quaint, primitive, charming, now that so few people live in rooms like this any more. And Campin's pictures, which once hung in houses very much like the ones they represent (these were devotional paintings intended for private use), now hang in a modern, air-conditioned art gallery. Part of their meaning has been dissipated, with their severance from the real, disappeared world that Campin's minute realism documented.

Yet Campin's realism is not simple. The tilted perspective of his paintings, which tends to pitch the background towards the viewer, and the minute distinctness with which everything in each picture has been delineated, have a powerful effect. This is, precisely, an effect of transfiguration. Campin's methods combine to create a sense of things observed with such fanatical closeness, and brought so close to the viewer by the attention paid to them, that they transcend their apparently straighforward nature. Everything, in Campin's paintings, seems to exceed its own ordinariness, to appear charged with a kind of curious, otherworldly clarity. Objects seem haloed (and hallowed), as if the sacredness of the mother and child has somehow irradiated their environment.

Things, in Campin's miniaturised world, acquire the quality of visions. A candle hung on a wall, a basket of linen, a plumped cushion, a length of hanging cloth: these objects, as painted by Campin, acquire the mysterious vividness and portentousness of things seen in dreams. This can partly be accounted for iconographically, and it is possible that the objects in these paintings seem as sacred as they do because of their connotations: the ewer and basin are objects used by the priest during the celebration of Mass; the lit candle and the crackling fire are ancient symbols of religious illumination; the basket of linen may even recall the basket in which Moses was laid by his careful mother.

But still these objects seem much more than prompts to dry symbolical exegesis. There is something about the way in which they have been painted - their isolation and presentation like precious things to be held in the palm of the hand; the way the light lies on them or is reflected off their surfaces - that makes them seem so out of the ordinary, so present and radiant and affecting. These objects seem sacred because they have been filtered through art: they have been subject to the transforming and never entirely explicable alchemy of painting.

Campin painted his two versions of The Virgin and Child in an Interior at a time when the expressive range of fluid oil paint was just being discovered by European artists. This may also partly account for the strange oneiric power of these two small tablets of painted wood. The National Gallery version, in particular, seems filled with a sense of pictorial discovery, of joy in the act of representation, which manifests itself most remarkably in Campin's painting of light.

There are three light sources in the picture, the candle, the fire and the window, and Campin has reserved his greatest delicacy of handling for the last: the dappled patch of paint to the immediate right of the shutters conveys the passage of sunshine on to a wall through the small panes of a thickly glazed window with a subtlety and virtuosity worthy of Vermeer. Campin's depictions of miracle are, simultaneously, invested with a sense of the miracle of depiction.

The National Gallery also currently contains four paintings by Pierre Bonnard, on loan from the Tate. They too are paintings of apparently simple occurrences in apparently ordinary interiors. Coffee is being drunk, someone is standing on a balcony looking out across town into the countryside, a table has been laid amply with platters of fruit and cheese. But here again, representation is the pretext for transfiguration, and Bonnard's paintings too are filled with a sense of the sacredness of the everyday.

Bonnard's sacredness is less specific, less bolstered by iconographic reference, less tied to religious ritual and story and altogether more personal than that of Campin. But it is just as unmistakable. In his art, it is painting and painting alone that works its transfiguring magic on the real, everyday, humdrum world: painting that melts and dissolves forms into paradise gardens of radiant, saturated colour, that invests dumb objects seen and domestic episodes experienced with the quality of visionariness. Bonnard's style is infinitely softer than Campin's, the objects that float across his canvases far more amorphous and harder to figure out, but in his art it is light, again, that gives the world its epiphanic quality: a dreamy mid-day light, the light of heat-struck stupefaction, whose glare turns things into visions.

Bonnard, like Campin, even includes the odd iconographic clue to his intentions. The young girl in The Bowl of Milk who stands, frozen, her face eerily lit, looks as though she is attending some mysterious votive rite. Something sacred must be taking place - but no, she is only giving the cat something to drink. For Bonnard, painting does not have to serve religion, to give form to its mysteries, to works its effects. Painting has supplanted religion, or rather painting has remembered its ancient similarities with religion. The ordinary has been made to appear extraordinary.

See Listings, opposite, for details.