A sonogram of baby Jesus

Three virgins, two of them without sin, one of them God.

That, in a nutshell, is the orthodox description of the Holy Family by the Catholic Church. Stated that way, the description appears vaguely outrageous and comic, a satire of orthodoxy in the making, if not a spoof of religious belief.

But the Holy Family, understood as icon - a silver-screen on which we view a mystery that gnaws at us - is a challenge to Catholic conservatives and progressives alike.

There is the refugee couple, a conspiratorial partnership against convention that preferred flight to submission, and their baby, their wailing, bubbling, leaking, messy future. Artists have made much of this icon, playing with light, shade and geometry, projecting on the nativity scenes a vision of a world pregnant with the future. In every age the best artists made the scene contemporary.

To find this icon interesting is to ask, among others, two questions about ourselves: Why are we touched by paintings (and stories and songs) of a baby born under threat, on the run? How can the Nativity, the feast of God made flesh, be explored in the biotech age where the human genetic code has been mapped and flesh is industrialised?

Conservatism is no refuge. It faces the insuperable problem of what to do with Joseph, Mary and baby Jesus as a model family once dogma so evidently implies that it is impossible for ordinary families to imitate them.

Progressives might seem to have an easier task - that of criticising a conservative tendency in the Church to contrast the Holy Family with ordinary sinners, to give them prerogatives based on their sexless chastity - a vision open to the criticism (repeatedly made) that it is steeped in hatred of the flesh and, especially, sex.

Yet, Joseph and Mary are a challenge to them too. Open to our patronising smiles because of their sexlessness, they pose, on second thoughts, for a powerful picture of the ideal contemporary couple, held together by the pleasure they find in each other.

What pleasures did the sexless Joseph and Mary find in each other? The traditional story is mute on this (as it is mute on their sexual life once the baby was born). But the story is quite clear that Joseph stuck to Mary because of a dream - another way of saying that he had an unstoppable technicolor conviction that Mary was what Joseph wanted, the heart of the world for him.

Like Joseph, Mary remains a mystery. The narrow icon of the submissive maiden virgin was a post-mediaeval invention, particularly developed in the 19th century. Christianity badly needs contemporary icons of the Holy Family. For our knowledge of the human genetic code tells us that our very bodies are storehouses of archived genes that are part of evolution's junkyard.

Our flesh is home to bacteria that are billions of years old, germs present at the start of life itself and important to our individual lives. The grainy sonogram on which most European parents first see their children is a picture of a relationship between new life and (we increasingly understand) not just the mother's womb but also her heart and mind.

For Christians, these themes seem ripe for interpretation and reflection. The link between the birth of one baby and the dawn of creation, the interface between all creation as divine communication, the repetitive, cosmic history prefigured in the story of one Jewish boy: these are Christian themes that ask for the story of divine involvement in the world to be told in our biotech language.

In the Middle Ages, for all the libellous things we say about people then, seemed to have got some things right. They made the connection between the birth of Jesus and the messy materiality of their world. They represented Mary in many ways - the images of mother and child were as important as the crucifix; they were gentle, fertilising, sexual images, showing the materiality of motherhood (such as breastfeeding).

We desperately need artists to paint sonograms of baby Jesus - a grainy shadow for us to peer at and guess its the divine potential.