Silver plate showing Shapur II (fourth century AD), made in Iran

Many of you will recognise the resounding opening music of the film '2001: A Space Odyssey' - Richard Strauss's symphonic poem, 'Thus Spake Zarathustra' - but these days most of us are pretty unsure about what Zarathustra actually did speak, or even who he was. Which is perhaps surprising, because Zarathustra, or, as he is more widely known, Zoroaster, was the founder of one of the great religions of the world. For centuries, along with Judaism, Christianity and Islam, it was one of the four dominant faiths of the Middle East. It was the oldest of the four, the first of all the text-based religions, and it profoundly influenced the other three.

There are still significant Zoroastrian communities all over the world, especially in the religion's homeland, Iran. Indeed, the Islamic Republic today guarantees reserved seats in its parliament for Jews, Christians - and Zoroastrians. In the Iran of two thousand years ago, Zoroastrianism was the state religion of what was then the Middle Eastern superpower.

My object today is a dramatic visualisation of power and faith in that Iranian empire. It's a silver dish from the fourth century, and it shows the king apparently out hunting - but in fact, he's keeping the world safe from chaos.

"This is the image of what the Persians call Shah n Shah - King of Kings. He rules because he is strong, because he is mighty, because he is powerful. And this was absolutely fundamental to the way that the Persians saw the functioning of the world." (Tom Holland)

This week's programmes are about ways in which people all over the world around 1,700 years ago found new ways of giving physical expression to religious belief, constructing images that would become so widespread that we're still familiar with them today. How those images came to be made, and the role that political and economic power played in spreading and sustaining these religions, are still matters of passionate debate.

In Rome at this time Christianity became the state religion, and almost contemporaneously, in Iran, the Sasanian Dynasty built a highly centralised state, in which secular and religious authority were bound together. At its height, this Iranian empire stretched from the Euphrates to the Indus - in modern terms, from Syria to Pakistan. And for several centuries it was the equal, and the rival, of Rome, in the long struggle to control the Middle East.

The Sasanian king out hunting on this programme's silver dish is Shapur II, who ruled with resounding success for 70 years, from 309 to 379. It's a shallow silver dish, I suppose it's about the size and the shape of a small Frisbee, but it's made of very high quality silver, and as you move it around you can see that it's got highlights in gold. The King sits confidently astride his mount, and on his head he wears a very large crown, with what looks like a winged globe on the top of it. Behind him, ribbons flutter over the silver, giving an impression of movement. Everything about his dress is rich - pendant earrings, long-sleeved tunic with carefully embroidered shoulder pads, highly decorated trousers and ribboned shoes. What we're looking at is an elaborate, carefully worked out ceremonial image of wealth and power.

So far, you might think that this is all pretty predictable stuff: kings have always shown themselves dominating animals, and overdressed. But as we'll see, this is much more than a conventional display of prowess and privilege. For the Sasanian kings were more than just secular rulers. They were agents of God, and Shapur's full titles emphasise his religious role: "... the good worshipper of God, Shapur, the King of Iran and non-Iran, of the divine race of God, the King of Kings". The god here is of course the god of Zoroastrianism, the religion of the state. Historian Tom Holland tells us about the great prophet and poet Zoroaster:

"Zoroaster is the very first prophet, in the sense that you would describe Moses or Mohammed as a prophet. No one is entirely sure when, or indeed if, he lived, but if he really did exist then he probably lived in Central Asian steppes in around 1000 BC. And gradually, over the course of the centuries and then the millennia, his teachings became the focus for what we could probably call a Zoroastrian church. And this increasingly became the state faith of the Iranian people, and therefore of the Sasanian Empire when it was established.

"The teachings of Zoroaster will sound very familiar to anyone who has been brought up as a Jew, or a Christian or a Muslim. Zoroaster is the first prophet to teach that the universe is a battleground between rival forces of good and evil. He is the first to teach that time will not go round in an endless cycle, but will come to an end - that there will be an end of days; there will be a day of judgement. And all of these notions have of course passed into the Abrahamic mainstream of Judaism, Christianity and Islam. The difference is that Zoroaster doesn't teach that there is one god. He teaches that there are two. There is the good god, and there is the evil god, and the whole of creation is divided up between these - even through the animal kingdom."

And it's when you come to the animal that the King is riding on the silver dish that you get a shock, because he's not on a horse but on a fully-antlered stag. He straddles the beast without either stirrups or saddle, gripping it by the antlers with his left hand, while his right hand deftly plunges a sword right into its neck - blood sprays out, and at the bottom of the plate we see the same stag in the throes of death. This whole image is quite clearly a fantasy, from the great crown at the top which would quite clearly have fallen off if you'd been riding, to the idea of killing your own mount in full leap. We're in the realm of symbol.

What's really going on here? In the Middle East, for centuries, hunting scenes have been a common way of representing royal power. Assyrian kings, well protected in their chariots, are shown bravely killing lions, from a safe distance. But Shapur is doing something else. This is the monarch in single combat with the beast, and he's risking himself not out of pointless bravado, but for the benefit of his subjects. As protective ruler we see him killing certain kinds of animals, the beasts that threatened his subjects: big cats which preyed on cattle and poultry, wild boar and deer which ravaged crops and pastures. So images like this one are visual metaphors for royal power, conceived in Zoroastrian terms. In killing the wild deer the hunter-king is imposing divine order on demonic chaos. Shapur, acting as agent for the supreme Zoroastrian god of goodness, will defeat the forces of primal evil and so fulfil his central role as king. Here's Guitty Azarpay, Professor of Asian Art at the University of California:

"It is both a secular image - because of course hunting was enjoyed by most people, by most nations and especially in Iran - it was also an expression of the Zoroastrian ideology of the time. Man is God's weapon against darkness and evil, and he serves towards the ultimate victory of the creator by following the principle of right measure, leading a life that is prescribed as having good speech, good words and good actions. In this way, the pious Zoroastrian can hope for the best of existence in this life and the best paradise spiritually in the hereafter. The best king is one who, as head of state and guardian of religion, creates justice and order, is a supreme warrior and a heroic hunter."

This dish is quite clearly meant not just to be seen, but to be shown off. It's an ostentatiously expensive object, made from a heavy single piece of silver, and the figures have been hammered out from the back in high relief. The different surface textures have been beautifully rendered by the craftsman, who's chosen different kinds of stippling for the flesh of the animal and the clothing of the King. And the key elements of the scene - the King's crown and clothing, the heads, tails and hooves of the stags, are highlighted in gold. When this was displayed in the flickering candlelight of a banquet, the gold would have animated the scene and focussed attention on the central conflict between the King and the beast. This is how Shapur wanted himself to be seen, and his kingdom to be understood. And silver dishes like this one were used by the Sasanian kings in vast quantities, sent as diplomatic gifts across the whole of Asia.

As well as sending silver dishes with symbolic images, Shapur also sent Zoroastrian missionaries. It was an identification of the faith with the state that was ultimately to prove very dangerous, especially after the Sasanian Dynasty was swept away and Iran was conquered by the armies of Islam. Here's the historian Tom Holland again:

"Well Zoroastrianism has really pinned its colours to the Sasanian mast. It has defined itself through the Empire and through the monarchy. And so when those collapse, Zoroastrianism is really crippled. And although over time it is accepted that Zoroastrianism should be tolerated, Islam never affords it the measure of respect that they give to Christians or to Jews. A further problem is that Christians of course, even those that have been conquered by Muslims, can look to independent Christian empires, independent Christian kingdoms, and know that there is such a thing as Christendom still in existence. Zoroastrians don't have that option, everywhere that is Zoroastrian has been conquered by Islam. And so as a result there is an increasing, sort of, spiral of decline to the degree that now in the land of its birth - in Iran - Zoroastrians are a tiny, tiny minority."

But if Zoroastrians today are relatively few in number, some of their faith's core teachings about the eternal conflict of good and evil, about the ending of the world, are still very powerful today. The politics of the Middle East are still haunted, and in some measure shaped, by belief in an eventual apocalypse and the triumph of justice - an idea that Judaism, Christianity and Islam all derived from Zoroastrianism. And when politicians in Teheran talk of the Great Satan, and politicians in Washington denounce the Empire of Evil, one's tempted to point out that: "Thus Spake Zarathustra".

In the next programme, we move from Iran to Britain of almost exactly the same date. From today's silver dish that symbolically combines power and faith at the heart of empire, I shall be with an object that offers us a very different view of sacrifice and redemption. We'll be in Dorset... with one of the earliest representations of the face of Christ.