Icon of the Triumph of Orthodoxy (painted in the late fourteenth century), from Istanbul

What does a great empire do when it's faced with imminent invasion and destruction? It can re-arm at home and seek allies abroad, but more cunningly it can revisit its history to forge a myth that will unite the people and carry them through to victory. A myth that will demonstrate to everyone that their country has been specially chosen by God and by history to uphold justice and righteousness.

History re-imagined can be a very powerful weapon. When the Christian Byzantine Empire faced obliteration at the hands of the Ottoman Turks around 1400, it turned to its past, found an event that proclaimed its unique and divinely ordained purpose, and turned that into a national myth. The Byzantines promoted their myth in the most public media at their disposal - they established a new religious feast-day, and they commissioned a religious icon to mark it.

"This icon is a document of a political event. I felt immediately that it was similar to the evening news." (Bill Viola)

"This debate about images just rolls in Christianity. Sometimes the image lovers win, sometimes the image haters win." (Diarmaid MacCulloch)

This week our programmes are about how people use objects to get closer to their gods. And how states use religious imagery to help rally the public. For the Byzantine Empire around 1400, it had never been more important to seek divine help. The successor to the Roman Empire, the defender of Orthodox Christianity, and for centuries the superpower of the Middle East, the Empire had shrunk to a shadow of its former greatness. By 1370 it was no more than a minor state that extended barely beyond the walls of Constantinople - modern Istanbul. All its provinces had been lost, most of them conquered by the Muslim Ottoman Turks, who now threatened the city on every side, and even the survival of Orthodox Christianity itself seemed to be in question.

There was little hope of military help from further away. Two brave attempts from western Europe to send reinforcements had been catastrophically defeated in the Balkans. On several occasions the emperor himself travelled from Constantinople to the kingdoms of the west - even as far as London - to plead for money and soldiers, but to no avail. By 1370 it was clear that there was going to be no earthly salvation. Only God could help in a situation so desperate. These were the bleak circumstances in which the icon of the Triumph of Orthodoxy was painted. It shows the world of the Byzantine Empire not as it actually was, but as it needed to be if God was going to protect it.

'Icon' is simply the Greek word for picture, and this picture is about a foot (30 cm) high, in fact it is almost exactly the same shape as a laptop computer. It's painted on a wooden panel, the figures in black and red, the background shining gold. In the centre, at the top, we see two angels holding up a picture for veneration. The picture they're holding is the most famous of all Orthodox icons, and one particularly connected to Constantinople. Known as the Hodegetria, it shows the Virgin Mary with the Christ Child in her arms. The Hodegetria is being venerated by a host of saints, by the head of the Orthodox Church - the Patriarch - and by the Imperial family. Between them, they represent all Constantinople, temporal and spiritual. So this icon is a picture about the use of a picture, and it's a celebration of the central role that icons play in the Orthodox Church. This is how Diarmaid MacCulloch, Professor of the History of the Church at Oxford University, describes the function of an icon:

"The icon is like a pair of spectacles which you put on to see heaven. You're drawn through this picture into heaven, because Orthodox Christianity believes very strongly that you and I can meet the godhead, that we can almost become like gods. It's that extraordinary frightening statement that western Christianity is very shy of."

The painting of icons was primarily a spiritual rather than an artistic activity, and it was governed by strict guidelines. The particular artist is not important, what's key is motivation and methodology. It's an aspect of icons that fascinates the American artist Bill Viola:

"Ok, this is a short text that I found in a book, and it's a text from the Middle Ages, and its called the 'Rules for the icon painter'. 'Number one, before starting work make the sign of the cross, pray in silence and pardon your enemies. Two, work with care on every detail of your icon, as if you were working in front of the Lord himself. Three, during work, pray in order...' "

And so the tasks go on - always emphasising that the artist's principal obligation is spiritual:

" '... Nine, never forget the joy of spreading icons in the world, the joy of the work of icon painting, the joy of giving the saint the possibility to shine through his icon, the joy of being in union with a saint whose face you are painting.' "

But what exactly is the Triumph of Orthodoxy, as shown in our painting at the British Museum? To find out, we've got to go back about seven hundred years. Given the centrality of icons in Orthodox worship, and the fervour with which they're described, it comes as a shock to discover that for a hundred and fifty years they were not only forbidden in Orthodox churches, but actively sought out and smashed. Around the year 700, the Byzantine Empire nearly succumbed to the armies of a new faith, Islam. In striking distinction to Christianity, Islam forbad the use of religious images, yet it was clearly an alarmingly successful faith. Had Christianity taken a wrong turn? Was it breaking the Second Commandment - the one that forbids the making of graven images? Was the state Church on the wrong road, was that why the military campaigns were going so badly? Suddenly, the use of images in church seemed to raise a huge and fundamental political question. Here's Diarmaid MacCulloch again:

"Can you picture God, or can't you? The huge dispute in the Byzantine Empire is one of those classic instances where that simple question is debated and becomes an issue which is actually very political, it splits the Empire down the middle. What happened in the Byzantine Empire was that it met an extraordinary trauma, which was Islam. Came from nowhere and smashed the Empire into smithereens, and naturally the Byzantines wondered, 'What's this all about? Why is God favouring these Muslims who have come from nowhere?' The one big thing that struck them about Islam was that there were no pictures of God, and that this might be the answer. That if you turn Christianity away from having pictures of God, then the Byzantine Empire might get God's favour back, and that seems to be one of the big motives in attacking images, icons, within the Byzantine Empire."

So a great wave of iconoclastic violence swept the Orthodox Church in the years following 700. The theological debates went on for well over a century, and were very complex. But throughout, the people remained on the whole very firmly attached to their icons, and eventually, thanks in part to support from the women of the Imperial family, the veneration of icons was restored in 843 by the Empress, Theodora. And it's this event that is known as the Triumph of Orthodoxy. It re-established the veneration of icons as the defining touchstone of true Orthodox faith, the central focus of Byzantine devotion, a vital ingredient in the flourishing and the survival of the Empire. And indeed the Empire did flourish, and for another five hundred years was able to keep the Islamic threat at bay. So when that threat returned even stronger than before, it was natural for the leaders of Constantinople to encourage the people to look back to that great moment of 843, when the faith had been reordered and the Empire restored, and to draw comfort from the past as they faced a frightening future. In 1370 the feast of the Triumph of Orthodoxy was established in the Church, and some time after that our icon was painted.

It shows us the Empress Theodora and that great restoration of 843. She stands beside the Hodegetria image of the Virgin and Child, and with her is her child, the boy Emperor Michael, both of them wearing elaborate Imperial crowns. Below them, in the bottom of the picture, stand a line of 11 saints and martyrs, crowded together as if they're posing for a group photograph, some of them holding icons in their hands like prizes that they've just been awarded. Any viewer around 1400 would have known at once that all these saints had suffered in the great struggle to re-establish the use of icons. All of them are neatly labelled, with their names written in red paint. My favourite is the one on the far left. She is St Theodosia, the only woman in the group and a feisty nun, who was put to death essentially for killing a policeman. She saw an Imperial guard climbing a ladder to remove an image of Christ from the entrance to the Imperial palace, she pushed away the ladder, and he fell to his death. Needless to say, she was promptly executed.

What the viewer around 1400 might not have realised is that some of these saints and martyrs were not even born in 843. The icon of the Triumph of Orthodoxy shows a whole society revisiting its past through a work of art, begging God to secure its future. It is, I think, a powerful and poignant image. Here's the artist Bill Viola again:

"It is an extraordinary and innovative picture, which represents a really ingenious way of uniting the temporal world of the past, present, future, with the eternal and the divine. When I look at it, I feel it's almost a post-modern-like image - using the idea of the frame within the frame, there are icons within the icons, images within the image."

The Triumph of Orthodoxy did not secure the survival of the Byzantine Empire. In 1453 the city fell to the Turks, Constantinople became the capital of the Ottoman Empire, and the great cathedral of Hagia Sophia became a mosque. The world's balance of power had changed. But if the Byzantine state foundered, the Orthodox Church survived. The faith we see proclaimed in our painting was strong enough to ensure that, under Muslim rule, the traditions of Orthodox Christianity, with the veneration of icons as its defining feature, endured. In one sense we could argue that this icon did exactly what it hoped: Orthodoxy survived, and every year, on the first Sunday in Lent, the Orthodox Church throughout the world celebrates the event shown in our icon, the Triumph of Orthodoxy. Here, as in so much of Orthodox worship, the image and the music of the human voice come together, in what for many is a powerful, and indeed overwhelming, expression of spiritual yearning.

In the next programme we're with a culture that has no problem with religious images, graven or otherwise. We're in India. I'm going to be with two of the great deities of Hinduism... Shiva, and Parvati, his wife.