But, the passing of the year 1000 demonstrated that Armageddon - although as inescapable a fact as the nuclear bomb in the late 20th century - was not worth holding one's breath for.
A monk at Cluny, writing three years after the great non-event, said: "It was as if the whole world, having cast off its age by shaking itself, were clothing itself everywhere in a white robe of churches."
For what was to follow the non-appearance of Armageddon was the biggest building boom in Western European history. A new sense of confidence, aided by financial growth throughout Europe, caused a ''white robe" of churches to be spread across the continent. It was time for serious ''thank you'' offerings, in the form of human approximations of the Holy City which had been predicted by Revelations, a City which "shone with the glory of God''. The first ''thank yous'' were abbey churches built in the solid, round-arched Romanesque style, as part of the monastic revival. But the millennial building race really took off about a hundred years later when the flamboyant Abbot Suger started redesigning the royal abbey of St Denis, north of Paris, in "the French style": his innovations included knocking down the dark forests of pillars and cramped walls of the original abbey, to make way for a lighter, more open and spacious interior; and making the effect of coloured glass windows (mainly sapphire blue, ruby red and emerald green) central to the experience of entering the church. The impact of these innovations was extraordinary.
The historian Jean Gimpel wrote in The Cathedral Builders (1980) that ''in three centuries - from 1050-1350 - several million tons of stone were quarried in France for the building of 80 cathedrals, 500 large churches and some tens of thousands of parish churches. More stone was excavated in France during these three centuries than at any time in Ancient Egypt.'' By the end of the Middle Ages, Gimpel estimated, there was a church or chapel for every 200 inhabitants in Western Europe: Amiens Cathedral was so huge that the entire population of the city could go to the same service; the vaults of the choir at Beauvais were so tall, at 150 feet, that a modern 14-storey skyscraper could easily fit inside. Indeed, they were so tall that they fell down in 1284. Each of these buildings - or sighs of relief in stone - was intended to be an encyclopedia of the latest thought, art and engineering; an experimental laboratory where geometry plus craft techniques could be tested out; they were facsimiles of the heavenly city itself, - and above all, they were an exemplar of the new architecture of light.
There was no millenium commission to oversee all this, but there was a convincing theological justification for boldly constructing what no man had constructed before. Abbot Suger wrote an inscription about it, to commemorate the rebuilding of the east end of St Denis:
Bright is that which is brightly
coupled with the bright,
And bright is the noble edifice
which is pervaded by the new light ...
This "new light", or claritas, could be interpreted at three levels: the actual lighting conditions made possible by the new stone and glass architecture; the light of the New Testament as opposed to the Old; and the idea (derived, it was claimed, from the writings of St Denis himself, the patron saint of France) that light was the first great principle of life. As Suger wrote, it was light, refected through glass or onto precious surfaces which transported him on a trip "to some strange region of the universe between the slime of earth and the purity of heaven". In fact, this new theology of light was based on a bizarre confusion between the French St Denis and two other ''Denises'' - one a disciple of St Paul and the other a fifth-century Syrian writer. If it hadn't been for this triple mistake, the French style could never have happened, for it would have had no basis in theology and no acceptable pedigree.
None of this could have happened, either, without money, and it was the rapid rise in the money economy that fuelled the ecclesiastical building boom. Most specifically, since national lotteries were still the best part of a thousand years off, the money came first from the bishop and chapter, through investments, profits from agriculture, mining, local industries, trade fairs (which took place during religious festivals), and regional taxes. Religion and economics were intimately connected in the late Middle Ages. Otto Von Simson concluded in The Gothic Cathedral (1956): "the religious sentiment that demanded so great an undertaking also provided the means for its realization." In the case of Chartres - a particularly wealthy diocese - the chapter agreed to commit all its extensive revenues for however long it took to build the new cathedral, except what was required "for subsistence". When the money ran out, the sacred relic around which the church was to be built could be sent on a fund-raising tour around western Europe. Emperor Baldwin II of Constantinople offered the Crown of Thorns as security for a substantial loan, while visitors to the shrine of St Thomas a Becket at Canterbury produced a quarter of the cathedral's entire annual revenues. Chartres possessed a particularly powerful relic - the Sancta Camisia, widely believed to have been the tunic worn by Mary when she gave birth to Jesus - which went on a benefit around northern France and southern England when building- work had to shut down through lack of funds. And it worked.
But cathedrals were also the engine of the local economy. And so financial contributions from feudal counts and dukes, merchants, craft brotherhoods and tradespeople - plus gifts in kind, and free skilled labour - were also substantial. Not exactly ''matching funding'', but something like it. Of the 176 stained glass windows in Chartres Cathedral, 43 were donated by the merchants and craftspeople of the town: their generosity was duly inscribed on the glass itself. The bottom line - literally - of the famous ''Good Samaritan'' window in the south aisle contains a message from the sponsor - the shoemakers. Panels show shoemakers at work and in one they can be seen offering the window to the cathedral. Shoemakers, of course, had a vested interest in the pilgrim business - it was a good way to wear out shoes - and they would also have gained redemption points for their generosity. But the motivation wasn't just, or even mainly, air miles. The cathedral was an integral part of everyday life: inside its walls from the evidence of various edicts, people would gossip, exercise their dogs, parakeets and falcons, play ball games, go courting, shelter from the rain, and buy wine and food from vendors who had set up shop all along the nave. The floor of Chartres has a gentle slope to it, probably to make the place easier to sluice out after pilgrims had turned it into a mass bed and breakfast.
There weren't any feasibility studies, or even detailed scale designs. During building, there was disagreement about precise units of measurement. Close examination of cathedrals built in the French Style reveal that different work-teams had very different standards of workmanship and even different ideas about design within the same building. The fate of the vaults of the choir at Beauvais demonstrated that the masons knew much more about geometry and stone-cutting than they did about engineering. And as for deadlines, the construction could take anything from 25 years (if the foundations already existed) to several generations: some cathedrals are still unfinished. What the builders did have, though, were amazing practical skills, with fairly rudimentary tools, the ability to put up a building which depended on stability rather than strength, and of course, the conviction that what they were doing was right. As a parish priest from Andres in southern France wrote around 1200 of a local building site: "who but a man stupefied or deadened by age or cares, could have failed to rejoice in the sight of that master Simon the Dyker, so learned in geometrical work, pacing with rod in hand and with all a master's dignity, and setting out hither and thither, not so much with that actual rod as with the spiritual rod of his mind?"
Renaissance commentators, a quarter of a millennium later, were to call the French Style Gothic - which was intended as a term of abuse (deriving from the word Goth or Barbarian). It was barbarian, they argued , because there wasn't enough ornament, and because the architect-hero had not put his name and his ego in capital letters all over the building. And yet, the achievement of the first millennium projects wasn't thought at the time to be stylistic at all. ''Gothic architecture'' was more than just a matter of flying buttresses or pointed arches or cross-ribbed vaults or dizzying heights. These were means not ends. The ends were a set of beliefs which expressed themselves in the new use of light and geometry - in short, a form of see-through architecture which was a machine for worshipping in. It was not so much a question of form following function, as of form following faith. And, of course, an expression of gratitude for surviving the first thousand years.
! Christopher Frayling is Professor of Cultural History at the Royal College of Art. His five-part TV series, 'Strange Landscape - the Illumination of the Middle Ages', continues on BBC 2 on Saturday, 8pm, and is accompanied by a book of the same title (BBC Books, pounds 18.99).