The Minutes and Papers of the Westminster Assembly 1643-52  (five volumes)
Chad Van Dixhoorn, editor
Oxford University Press £630
(978-0-19-920683-4)
Church Times Bookshop £567 (Use code CT261 )

TO LOOK at it through one eye, it is astonishing that we have had to wait until now for a book that might well have been published two centuries ago. To look at it through the other, it is astonishing that it has been managed at all.

Although £630 may make Church Times readers wince, it is a fair price for a decade of the editor's life. This set will now be utterly indispensable: every research library should buy it. It will be used into the next century and beyond.

The Westminster Assembly is little-remembered nowadays in the country that hosted it, but its most famous product, the Westminster Confession, is the touchstone of orthodoxy for tens of millions of Presbyterians worldwide. As well as the Confession and a handful of other key documents, however, the Assembly left behind it a vast (though far from complete) record of its deliberations: more than half a million words in, as it happens, execrably bad handwriting. Scholars have periodically dipped into this daunting and almost incomprehensible morass. Now Chad Van Dixhoorn has laid the whole thing bare for us.

The Assembly was a kind of Civil War-era equivalent of the Beveridge committee. In 1643, while the war between King and Parliament was in no way decided, Parliament set up an Assembly of Divines, whose purpose was to frame a post-war religious settlement. Its members were chosen by Parliament, which might not seem an obvious way to do theology. Richard Baxter drily commented that the Assembly was formally simply an advisory body to Parliament, and that "the Parliament . . . did think that they best knew who were the fittest to give them Advice, and therefore chose them all themselves."

In fact, a few were chosen by the Scots, following the alliance between the English Parliament and the Scots Covenanters: this made the "Synod of London" into an international body and gave it some of its enduring authority (although history and tradition has inflated the Scots' contribution).

More importantly, however, both the English and the Scots chose well. The Assembly's membership is a roll-call of the great theologians of the time - or, rather, of the great theologians who sympathised with the Parliamentary cause while still favouring a comprehensive national Church. They were bolstered by 30 MPs, who were there to keep the Assembly on the rails, and whose presence rather embarrassed their clerical colleagues. The Assembly's effective president and business manager was the minister Cornelius Burges, a function that makes him perhaps the most important English clergyman of whom you have never heard.

It remains one of history's great might-have-beens. If the Civil War had been won more quickly, and if the King had been willing to negotiate rather than keep shedding his subjects' blood in his quarrels, the Assembly could have set Britain's religious direction down to the present. It was clear from the beginning (the Scots' participation virtually guaranteed it) that the Assembly would recommend a Presbyterian structure of some kind. The Parliament and the Scots were committed to "root-and-branch" extirpation of "prelacy"; both wanted to maintain a single national Church. The Assembly contained plenty with qualms on either side, but the direction of travel was unmistakable.

And yet it was not to be. No one wanted bishops, who by then were seen as little more than mitred tyrants. But the Parliament did not share their divines' purist Presbyterian zeal. The legislation for a Reformed national Church from 1645 onwards did not grant the robust independence that Presbyterians demanded. All the structures were there: parishes would elect their own governing elders, who would elect presbyteries, who would in turn elect regional assemblies - but the regional assemblies would ultimately answer to Parliament. Presbyterians, temperamentally unsuited to seeing a glass as nine-tenths full, smelled betrayal.

In the event, however, their ambitions were sunk not by political perfidy, but by military radicalism. John Milton's famous accusation that the new presbyters were simply the old priests writ large was not widely shared: in the Assembly, the radicals who joined him in rejecting comprehensive national structures were a minority who could be voted down. In the streets, they could not be so easily suppressed, and no one had the stomach to do so. In any case, their base of support was in the victorious Parliamentary Army, which from 1647 onwards was plainly the locus of real political power.

And so the Assembly found itself moved to the sidelines. A national Presbyterian Church was erected in law, but not in fact: only in London and Lancashire were the regional assemblies ever properly set up. On one side, Congregationalists and radicals of all stripes - most of them more or less orthodox Christians, a few decidedly not - sprang up across the country. On the other, huge numbers continued to cleave to the banned Book of Common Prayer and to resort to the deprived bishops. We now know that in the Interregnum years more ministers received illegal ordination from a bishop than legal ordination from a presbytery (a few, but only a very few, covered all bases by securing both). The Assembly itself was finally extinguished, together with the Rump Parliament, in 1653.

Van Dixhoorn's vast project allows us, for the first time, to track this extraordinary story from within the Assembly's walls. It is a unique window on the period that is still England's defining trauma, and which (for good or ill) created the modern Church of England.

But, as with any edition, the joy of this one is in the detail as much as the sweep. The painstaking intensity of the theological debates that it records is inevitably moving. Intricate questions of polity are teased out in session after session: as a Reader myself, I find the agonised discussions about whether "ruling elders" are more closely aligned with the laity or with the pastorate oddly familiar.

And we also glimpse, repeatedly, a nation's revolution in progress. There are detailed reports on the attempts in 1643 to secure Londoners' consent to the Solemn League and Covenant. The Rector of St Olave's, Hart Street, absented himself (he said he had a baptism), and the congregation made "a noyse that savoured of some dissentient"; by contrast, at St Andrew Undershaft there was "a great congregation; many held up their hands & subscribed", especially "the midle sort". Events did not, in the end, unfold according to anyone's expectation. But we can now follow both the human and the theological drama as we never have before.

Dr Alec Ryrie is Professor of the History of Christianity at Durham University.