FOR A THOUSAND years the Arthurian legends have endured undiminished by progress or pessimism, and in this triumphant comic reaffirmation by Thomas Berger, they will continue to enthrall readers.

It scarcely matters that there was no historical King Arthur - or that if there was, he was likely a Welsh guerrilla of dubious gentility who made a name for himself fighting the Saxon invaders.

It doesn't matter because it is characteristic of the dreamy human animal to invent an idealized past. And when the chroniclers of the 12th century set out to propagate such an ideal, they appropriated the existing legend of Arthur. Thus the idylls were born begotten by the florid French imagination upon the durable mythology of the Celts, and subsequently invested with the curious, elaborate amalgam of Christianity, pagan blood revenge and woman-worship known as chivalry.

Berger's expansive novel subsumes all these elements and more, stretching from the ribald reign of Arthur's father, Uther Pendragon, to the young Arthur's first pretentious rule ("How to be righteous without being sanctimonious we see as our problem"), to the grand era of the Round Table. Then it moves gradually through the waning years - in which an aging Arthur fears that "evil-doing hath got more subtle, perhaps even to the point at which it can not properly be encountered with the sword" and the magnificent Launceiot goes idle "in his velvet house-coat and slippers" - to the rise of Mordred, followed by Arthur's epic death upon the Salisbury Plain.

Of course, to portray a mortal man in a mythic situation is to invite comedy. And as John Barth did in Chimera, Berger exploits the humorous human potential to the fullest, but without compromising the integrity of the original legends - Gawaine and the Green Knight, or the tryst of Tristram and Isolde.

Instead, Berger enriches the texture of the tradition by speculating on the background of each knight - Percival's upbringing as a sissy in girls' clothes, Gawaine's fantastic carnal appetites, Launcelot's ascetic monasticism ("Methinks that fighting evil," he says, in his hair shirt, "is but finally to give it a reputation which unaided it could not aspire to").

The familiar tales are told in a style as deliberately atavistic as that employed by the translators of the King James Bible in 1611, and to the same purpose: to give the whole a venerable aura and impact. Berger's 15th-century syntax, although inconsistently sustained, succeeds in giving the book both a self-mocking playfulness and a seeming gravity, according to his needs.

But even an imaginative retelling in antique language is less than Berger intends. As Tennyson asks in his Morte d' Arthur, "Why take the style of those heroic times? / For nature brings not back the mastodom, / Nor we those times; and why should any man/Remodel models?"

Various authors have answered that question variously. Sir Thomas Malory in the 15th century may have intended the tales to justify his own violent life (his Morte d'Arthur was completed during his years in prison); and many feel that Tennyson bent the Idylls of the King into an encomium to progress ("The old order changeth, yielding place to new, / And God fulfills himself in many ways, / Lest one good custom should corrupt the world"). T.H. White, pacifist author of The Once and Future King, who once thought Malory's chronicle "a regular greek doom, comparable to that of Orestes," had decided by 1940 that "the central theme . . . is to find an antidote to war."

But Berger's world of the Round Table and its comic-heroic exploits is more obstinately complex, less susceptible to moral redaction. He knows that all true myths are ritual reenactments of timeless human dilemmas, endlessly suggestive, ultimately inscrutable.

Thus his very human heroes constantly find themselves trapped in ambiguity, and live their comic lives against a background of serious problems perplexingly unresolved: the nature of sin; the spirit versus the letter of the law; how to fulfil God's perfect will in man's fallen world; and how to maintain the artificial virtue of civilization without relapsing into primitive patterns.

For example, when Arthur is crushed by Christian remorse at his unwitting act of incest with his half-sister Margawse (the issue of which is Mordred, who eventually will kill him), he confesses his sin to Merlin. "Well," the sorceror replies, "what are crimes to this religion of Jesus of Nazareth are of indifference to Nature, Sire, and though I expect you shall find me blasphemous, let me say that Nature was here first and will be here last."

But the Lady of the Lake ("I am interested only in that which is mythical") has yet another attitude toward virtue. "All human beings must perform according to their nature," she says. And when Arthur asks, "Then the will is not free?" she responds that "This is the wrong question, being political and not concerned with the truth."

Other characters find other ethics. Old King Mark of Cornwall, "who had been a good king for a while . . . came to reflect that being kind and decent had had no effect on the events of his life, and . . . he henceforth determined to be bad again." And Guinevere wonders in the end "whether those who were not knights did not have it better, living according to their appetities, for the common folk and the beats fought only for food and sometimes their lusts, and being a woman she could not understand honor and justice, for they were invented by men."

Finally, the reader can only sympathize with Launcelot in his confusion: that if living is full of moral complexities, "it was because chivalry in general was more complicated than it seemed, for it is not easy always to know what is the noble thing, or what is brave and generous or even simply decent."

By having the confidence to leave an evocative story alone, and the courage to elaborate when appropriate, Thomas Berger synthesizes the disparate Arthurian romances into a splendid, consistent narrative - and makes them speak eloquently to a modern audience.

Moreoever, he even provokes an unexpected nostalgia for that imaginary age in which honor was at stake in every daily act: "for all men of that time lived and died by legend (and without it the world hath become a mean place)."