The National Gallery's groundbreaking exhibition displays altarpieces as more than just works of art

Art galleries are unintentionally confusing places because feet of wall space are devoted to religious paintings taken out of context. If you compare, say, a Counter-Reformation altarpiece in a Baroque church still in the position for which it was painted you will find a complementary architectural framework that makes sense of it as part of a liturgical composition. Divorced from that setting it becomes an easel painting on its own terms that was never intended to be seen as such.

Devotion by Design does much to rectify this problem because for the first time in this country, altarpieces are exhibited with simulated altars attached and mounted at the height of their original positions. Some are displayed as free-standing compositions in order to show the carpentry which is normally hidden when they are placed against a wall. Few, including art historians, think of the surfaces and structure on which the altarpieces were painted and it is illuminating to discover how important they are. New ground has been broken by emphasising this element which should, of course, be obvious but isn’t.

Reredoses have become unfamiliar pieces of church furniture in the last 40 years due to the post-Vatican II practice of celebrating Mass facing the people on westward-facing altars with nothing behind except, perhaps, a large crucifix or a tapestry that looks as if it is going up in flames. In the days when Mass was celebrated ad orientem the worshipper would be faced by one endowed with rich iconographical symbolism that sets the action of the Mass within salvation history and the Communion of Saints. They should not merely be treated as works of art. Not long ago Cardinal Cormac Murphy-O’Connor reminded us that Piero della Francesca’s Baptism of Christ is “a work of faith and piety, an expression of the Church’s life and a way into prayer”, and not merely a painting. Piero’s and many other masterpieces from the National Gallery’s permanent collection of Italian altarpieces before 1500 comprise the greater part of the exhibition and it is free. But it begins with a room of 15th-century Flemish panel paintings including The Mass of St Giles and Rogier van de Weyden’s The Exhumation of St Hubert. They represent with meticulous attention to detail the celebration of Mass in order to provide a liturgical context for what follows.

Devotion by Design progresses by examining different types of altarpieces such as the polyptich (a picture, or relief, made up of several parts) and pala (a large version used only for a single picture). This is followed by a room devoted to how they came into being: commissioning, choices and contracts. In the section devoted to sacred space an attempt is made by way of explanation to reconstruct an altar. Over time many altarpieces have been dismembered and their fragments dispersed and evidence of that is found in a part devoted to dislocations, notable for the work of Fra Filippo Lippi. It concludes with a final room obscurely entitled “a question of definition”. The result is a total immersion in the subject represented by paintings of great beauty.

It is a cliché to describe the Church as one of the most creative and destructive influences on western civilisation. This phenomenon is associated with the development of doctrine and liturgical practice. Scott Nethersole, in the excellent book accompanying the exhibition, questions the plausible assumption that reredoses came into being as a result of the definition of the doctrine of transubstantiation promulgated by the Fourth Lateran Council in 1213; the Council also legislated the presence of candles and a crucifix on the altar. But he does so on the basis of private reservations held even by saints like the Poor Clare, St Catherine Vigri. Her doubts were resolved by an apparition of Christ who personally explained the mystery. From this time came the elevation of the Host made visible to the congregation against a background of increasing splendour.

Altars later came to combine several functions to make room for a tabernacle for the reservation of the Blessed Sacrament with a crucifix above and a gradine for candlesticks. Eventually the size and importance of the altarpiece frequently overshadowed the altar itself and replaced it as the focal point of the church, as in Titian’s Assumption in the Frari church in Venice. This was done at the expense of old altarpieces which became expendable and subject to depredation, hence the multiplicity of fragments in the art galleries of Europe. Sienna, Montepulciano and other northern Italian cities mercifully still retain some of their medieval masterpieces. London has become the centre for exhibitions of Catholic art this summer. There are the relics at the British Museum and this equally outstanding show at the National Gallery. Both are complementary and are recommended without reserve.

Devotion by Design is on at the National Gallery, London, until October 2