Ingres, Jean-Auguste-Dominique : Madame Paul-Sigisbert Moitessier, Standing (1851)

"Schematic" is a critic's rude word. It's something a work of art shouldn't be. "Schematic" and "formulaic" and "diagrammatic" are ways of saying that a work is too neat, too rigid. Schemes and formulas and diagrams - these things may be all right in science. But when it comes to art they're all wrong.

Perhaps they're wrong for reasons of realism. Life and nature and the human heart are terribly messy. Art should not tidy them up too much. Or perhaps they're wrong for reasons of sheer entertainment. Being the messy creatures we are, we demand a bit of surprise. At the same time, there's such a thing as a beautiful diagram. And when you're trying to picture a point of religious doctrine, a good scheme may be just what you need.

The Christian doctrine of the Trinity beggars belief and description. The idea is one single God, but three separate persons - namely, God the Father, God the Son (Jesus) and God the Holy Spirit. Stick to those words, and you'll be OK. But almost every way of trying to make the idea clearer leads to incorrect deviations.

The three persons exist in a relationship, but they are not three different Gods. They are not three subdivisions of the one God. They are none of them more or less God than the others. Yet they are not just variant aspects, or roles, or avatars of God either. You must not say anything that makes God sound more than utterly single, or anything that makes his three persons sound less than distinctly separate.

The Trinity is perhaps easier to picture than to verbalise. Pictures are more flexible about identities. You can have a picture in which a figure appears twice, but is understood to be the same person, seen at different times. Likewise, the three persons of the Trinity can appear together, without any denial that they are also - in some sense - one.

The least controversial image is a pure diagram: an equilateral triangle, three distinct sides joined in a single form. There are also more realistic but symbolic ways of picturing it. There's one that shows the Father as a bearded man, holding up Jesus on the Cross, with a dove, the Spirit, hovering above them. There's one that shows a head with three faces.

But the most elegant solution is the one used by Andrei Rublev in his icon of The Holy Trinity: the Trinity here is an encounter, an act of communion, three human figures seated round a table. It's inspired by an episode from the Book of Genesis, which was traditionally taken by Christians to be about the Trinity. Abraham is visited by God - yet God seems to be a threesome. "And the Lord appeared unto him in the plains of Mamre... and he lift up his eyes and looked, and, lo, three men stood by him..." The visitors sit down. Abraham and his wife Sarah give them a meal.

Rublev's version economises strictly. He leaves out everything but the angels at their table. They sit there, with wings, halos, staffs, a cup between them. Behind them appear a house, a tree, and a rock. It's a scene designed for intellectual reflection. Interpretations are disputed. There is no agreement even on which figure is Father, Son or Spirit. But what's clear is Rublev's central pictorial formula for the Trinity. It's an economical and ambiguous composition that puts the three figures in a relationship that's both stable and dynamic.

The three are near look-alikes, and equal in size, but not identical in pose. They are sitting round their table in a ring, or equally in a triangle. But the way they sit means that each figure in turn can be singled out from the trio. The two figures on the right face and bow towards the figure on the left - that singles him out. But the two figures on the left are both seated with their bodies addressed towards the one on the right - and that singles him out. And the two outer figures can be seen as mirror images of one another, making a frame that singles out the central figure, who - turning in two different directions - mediates between them.

Or if you look at the heads alone, you can see a circulation of attention, so that the right figure bends towards the central figure, who then bends towards the left figure, who looks back across to the right figure, and so on. Or, following their torsos, you can take the relay round the table the other way.

Rublev's solution to the mystery of the three-in-one is not a matter of fusion or hierarchy. It's a matter of conversation. He devises a structure that sets up a continuous interplay among the three persons, so that each relates to the others in different ways, and no relationship dominates' so that each figure is simultaneously separated and integral.

THE ARTIST

Andrei Rublev (1360-1430) is the name that glows out most strongly from the often anonymous art of the Russian icon. He's just about the top Russian artist, until Malevich. He was a monk, and grew up in a monastery in Zagorsk. He is praised in the same terms as his early Renaissance Italian contemporaries - for introducing psychological subtlety, a sense of realistic space and volume, lyricism of design. Only a few icons from his hand survive, such as this one, considered his masterpiece, and much copied. Andrei Tarkovsky's 1969 film Andrei Rublev (above) is a vivid imaginary portrait of the artist and of medieval Russia.