The Scythians of Ancient Greece did not water down their wine.

This was a breach of Attic etiquette and was widely regarded, not least by censorious Athenians, as an uncivilised practice that led to sore-heads, quarrels, and radicalism. By the 1560s, the phrase "to drink like a Scythian" was still in common usage: short-hand for denouncing extreme behaviour. It naturally entered the mind of the English cleric John Jewel when he meditated on ecclesiastical events north of the border, though he twisted the phrase for his polemical purposes. Through his rapid and wide-ranging reforms, John Knox was "churching it like a Scythian." Knox and his cronies were getting theologically drunk on undiluted Calvinism.

Professor Jane Dawson, of the School of Divinity at the University of Edinburgh, reminds us of this memorable line and it does a good job of encapsulating the stock image of the most famous preacher ever to emerge from Scotland. Knox could be uncompromising to a fault and he was expert at denouncing those with whom he disagreed. By the mid-1540s he was a card-carrying Protestant and for the remainder of his life he was guided by certainties. As Dawson puts it, Knox was "happiest viewing the world in crystal-clear polarities." He saw a true church and a false church, and he regarded the outfit based in Rome as the Whore of Babylon. Anyone of reformed sensibilities who compromised with Catholicism was a wretch.

There are, of course, two ways of looking at all this. We can regard Knox as either an overzealous bully or as man unflinchingly committed to carrying through his project of purifying the Christian faith. Or perhaps, as Dawson's superb and even-handed study indicates, he was a bit of both. We can certainly conclude that Knox was consistent. He was adamant that Scripture should be the only lodestone of Christian worship and took aim at what he regarded as a millennium's worth of man-made doctrinal and liturgical inventions. He took this path during his 1550s continental exile, landing himself in a great deal of trouble in Frankfurt, and he followed it when he attempted to create an authentically reformed Scottish Kirk.

The Knox that emerges from these pages is pleasingly multifaceted. It does not suffice to see him as the "personification of the puritanical kill-joy." He was a man who laughed, loved deeply, and someone "to whom tears came easily." Knox worked very hard to cultivate his self-image as a prophet but a genuine concern for the future of his faith sat alongside the egotism.

Dawson captures all of this and her biography is likely to be regarded as definitive for a very long time. It draws on recent archival discoveries (notably the cache of papers related to Knox's best friend, Christopher Goodman) and is brimful of surprises. It is no secret that Knox and Mary Queen of Scots were not the best of pals, especially when Knox decided to sermonise about Mary's marital plans. It was not all rancour, however. At one point they teamed up to sort out the breakdown in the relationship between the earl and countess of Argyll, becoming an "unlikely pair of marriage guidance counsellors." Who knew?

The book also portrays Knox as a truly international figure. He will always be remembered, for excellent reasons, as the great Scottish Protestant but he was every bit as concerned about the religious future of England. We are informed that, following his time south of the border during Edward VI's reign and his spells in English exile communities, he "regarded himself as an Englishman by adoption." With the death of Mary Tudor, there was a chance to play this role with more vigour than ever before but Knox had recently made one of the "costliest mistakes of his career." In his famous The First Blast of the Trumpet against the Monstrous Regiment of Women Knox declared that female rule was against nature and, fatefully, he declared this to be a universal truth. The text was aimed squarely at Mary Tudor but it was never likely to impress her successor and half-sister Elizabeth. It was made very clear that Knox was not welcome in England.

In a way, this episode captures the essence of John Knox. He was not, by his own admission, a sophisticated or systematic theologian so when he located an appealing theme he hammered it home with scant regard for the consequences. Just think of that famous Perth sermon in May, 1559 that sparked an iconoclastic riot and is usually identified as the starting gun of the authentic Scottish Reformation. John Jewel therefore had a point. Knox did not dilute his theological wine but one suspects that he would not have objected to being called a doctrinal Scythian. He wanted to change the world and spat venom at anyone who challenged his vision. This was annoying and brave in roughly equal measure and it certainly made for one of the most fascinating sixteenth-century lives. Until now, we have usually had to make do with caricatures. Dawson has brought nuance and peerless scholarly rigour to a cautionary, but also rather inspiring tale. You will be familiar with that hackneyed question about which historical figure you'd most like to invite to dinner. I'd venture that John Knox should be close to the top of your list.

John Knox, by Jane Dawson (Yale University Press, £25)