As a sacred text, the Bible has a special and unpredictable power. To the oppressed, it can signify rebellion and freedom, yet to the powerful, it may provide a sure foundation for their governance. So in the Roman Empire, the books making up the New Testament, which came to be accepted alongside the Hebrew Scripture as part of the "Canon of Scripture," shaped the new religion which its enemies saw as subversive.

Then in the fourth century of the Christian era, when the Roman Emperor Constantine seized the destiny of Christianity within his boundaries, one of his gifts to his new religious ally was the slaughter of around five thousand cattle to provide the parchment for fifty huge de luxe copies of that same Bible.

From Constantine's time till the present, governments have looked to the Bible for its commands for subjects to obey their rulers, ignoring its other messages, while the poor have hearkened to the fact that Jesus Christ called them blessed.

In the United Kingdom, the Bible in English has experienced the same shift in fortunes. In the fourteenth century its translation was the project of rebels against the Western Latin Church, whom the mainstream population contemptuously nicknamed Lollards.

During the Lollards' struggle with the church authorities of their day, they turned the Latin Vulgate Bible into the language of southern England, deeming it vital that the humblest uneducated person should have the chance to read God's word.

When the Lollard movement was crushed, the leaders of the English Church, meeting in my own city of Oxford in 1408, banned the Lollard Bible. This ban was unique in medieval Europe, for in other countries there were many vernacular translations, but it persisted in England after Henry VIII's break with Rome.

As a result, in 1536 the translator of genius, William Tyndale, was executed in the Low Countries as a heretic by officials of the Holy Roman Emperor, with King Henry's connivance. Yet within only three years of Tyndale's death, Henry was authorising for his realm a Bible translation which was in large measure Tyndale's own: maybe the royal monster never realised how his enthusiastically evangelical chief minister Thomas Cromwell had hoodwinked him in this bizarre shift of policy.

Through the next seventy years, biblical translation bounced between officially-sponsored versions and alternatives produced by those at odds with the English monarchy - even, in the end, a Roman Catholic version.

But now in 2011, we celebrate the fourth centenary of an English Bible which was sponsored by a King, and has become so associated with him that its principal nickname has always been "the King James Bible" (the KJB).

It is appropriate that an unusually clever and scholarly monarch should be remembered by a book. James VI of Scotland and I of England used to have a bad press among English historians, caricatured by one of his early enemies as the "wisest fool in Christendom." He was in fact a highly intelligent man who enjoyed a good joke, and before coming to England in 1603, he had been a remarkably successful king of Scotland in frequently difficult circumstances.

He could be personally disconcerting. It has been plausibly suggested that some of his personal characteristics, such as his speech impediment and regular incontinence, are indications that he was born with mild cerebral palsy. His English subjects found difficult his breezy informality and his thick Scots accent under the speech impediment, and in any case they were uneasy at having a foreigner as king.

Yet James's (admittedly episodic) interest in good government created something of a golden age for the established churches in England and Scotland, and that was a stroke of luck for the Bible which his translation committees published in 1611. It may have lasted so long not so much through its literary merits (though it has many) but by the accident of the era in which it appeared.

King James's Bible was produced in a narrow window of opportunity in the 1610s, when the English and Scottish Churches were rather grudgingly moving together, and before English Protestantism irretrievably fragmented, during the civil wars which engulfed these islands in the 1640s and 1650s, provoked by the disastrous policies of James's son King Charles I.

Published thanks to a king who in retrospect appeared a model of Protestant commitment compared with his untrustworthy offspring, the KJB had the potential to become a uniting symbol for English-speaking Protestantism - and rather against the odds, that is what happened.

It was not tainted by Charles I; it did not become a totem of royalism, as so easily could have been the case, and indeed versions were printed under the aegis of Lord Protector Cromwell. By the time that the episcopally-governed Church of England came back with Charles II in 1660, even Protestants who so disapproved of bishops and the Prayer Book that they rejected the new Established Church, turned away from the "Geneva" version which their Puritan parents would have preferred, towards the new Bible.

Two giants among Dissenters who defied the new "Anglican" Church of England and suffered at its hands, John Bunyan and John Milton, habitually quote the KJB in their writings. England's majority population of Anglicans henceforth experienced two books which shaped their prose - the Prayer Book and the Bible - but English Dissenters and the Established Church of Scotland (which after 1690 also rejected episcopal church government) became people of a single book, the KJB.

It is hugely important that English and Scots combined in this. Now that England and Scotland jointly stumbled on a "British" world empire, the unifying book in the English language which they took to new lands was the KJB.

The Scots increasingly confined their own centuries-old northern forms of English to private use, since throughout the sixteenth century and thereafter, they read the Word of God in the language of London: if anything welded the two ancient enemies together, it was this joint possession of an English Protestant Bible.

Now its most ardent defenders are to be found amid the multiple Protestantisms which British emigration bequeathed to the United States. Some of them, "King James Only" folk, believe that it possesses an extra dose of the Holy Spirit not granted to any other English version, which is very generous of them, considering that it was commissioned by a monarch whose cheerful bisexuality would at the present day deeply shock them.

The history of the KJB after it escaped King James's England and Scotland is inseparable from the rise of the British Empire and the first great nation to escape British control, the United States of America.

During the later seventeenth century, KJB language, already self-consciously old-fashioned in 1611, seemed almost embarrassing, as a teenager feels embarrassed by Mum and Dad. In fact the KJB we read today is a tamed version from the eighteenth century, when in 1743 a Cambridge don, Francis Parris, published a vigorously-updated version of the 1611 text; this was only slightly modified in 1769 by an Oxford scholar, Benjamin Blayney, who has succeeded in taking most of the credit for the lasting modification.

The impulse to replace KJB language might have swelled into a new translation then, but the French Revolution convinced the British that foreigners were not just annoyingly foreign, but a threat to all that was good in our islands' national heritage.

It was around 1800, therefore, that the KJB took on its present iconic "heritage" status. The same mood canonised William Shakespeare and made him known wherever English was spoken - and both these works live on even in translation, beyond the sound of English.

The KJB lies behind a surprising variety of other Bibles created in the last two centuries. In the mid-nineteenth century, the Chinese translation of the KJB - along with the translation of another English Protestant classic, John Bunyan's Pilgrim's Progress - inspired a troubled failed aspirant to the Chinese civil service, Hong Xiuquan, to lead a great rising against the Chinese Emperor. Hong's Taiping Rebellion has a fair claim to be the most destructive and horrific civil war in human history.

So the Bible has continued its paradoxical role: it puts down the mighty from their seats, even though more than once, its translation has been commissioned by a monarch. Still the KJB echoes not just through churches, but through novels, plays, poetry, puns, jokes.

There is a possibility that Christian churches will use the commemorations of 2011 as a way of saying goodbye to it: a big service of thanksgiving, perhaps a year of taking a reading from it in services, and then, nothing more, turning to the welter of translations which have appeared in the last century - Revised Standard, New International, Good News, or a huge variety of other well-meaning efforts to make the text more easily comprehended.

Yet to leave the KJB behind would be a mistake. It is not easy for us to understand nowadays, and that reminds us of something very important about its contents: the Bible has emphatically never been an easy book to understand. It is not even a book, but a library of books, with a cacophony of voices in them. Its name in Greek underlines that, for it is in the plural, Biblia, BOOKS.

Long ago, Archbishop Thomas Cranmer made a point of warning bible-readers that they were doing something dangerous in reading Scripture, in the preface which he wrote in 1540 for Henry VIII's "Great Bible." Rather surprisingly, in a text designed to commend the Bible, he put a health warning on it. He likened it to things which were very good, but dangerous if used wrongly:

"What is there here beneath, better than fire, water, meats, drinks, metals of gold, silver, iron, and steel? Yet we see daily great harm and much mischief done by every one of these, as well for lack of wisdom and providence of them that suffer evil, as by the malice of them that worketh evil ..."

So whenever we open the covers of this library of books, we are playing with fire. We need to realise what power lurks within, and how difficult, exciting and perilous is the expedition on which we embark. King James knew this, and his translators may be wise and eloquent companions on our journey.

Diarmaid MacCulloch is Fellow of St Cross College and Professor of the History of the Church, Oxford University. His latest book is A History of Christianity: the first three thousand years (Penguin, 2010).