IT TOOK more than 400 years, but Raphael's reputation at last has declined. It could not possibly have risen. Today he is acknowledged as just a major master. Once he seemed divine.

He used to be regarded as art's apotheosis. "Fair in body, fairer in mind, charming in manner, admirable in art, unwearied in labor, eternal in glory" is how Joachim Sandrart viewed him in the 19th century. Eighteenth-century connoisseurs, Thomas Jefferson, for instance, esteemed his art as highly. Cultivated persons, and common folk as well, saw Raphael the pure Raphael the pious, as the greatest of the painters. The richest of the squillionaries yearned to spend their fortunes on Raphael's Madonnas. One of these sweet paintings, the "Madonna of the Chair," used to be more famous than the "Mona Lisa." Viewers in the Pitti Gallery in Florence, Henry James observed, approached that "lovely picture as a kind of semi-sacred, an almost miraculous manifestation. People stand in worshipful silence before it as they would before a taper-studded shrine."

Today they pause, then pass it by. Raphaels no longer seem to glow more brightly than works of art by Rembrandt or by Leonardo, or even by such lesser talents as El Greco. Some unpredicted change in taste -- in just the past half-century -- has whittled at his fame.

He was born 500 years ago, in the lovely town of Umbria, on Good Friday, April 6, 1483. "Raphael and America," which goes on view today at the National Gallery of Art, marks that anniversary. A scholarly exhibit, organized by David A. Brown, the Gallery's curator of early Italian painting, it has a dual mission. Raphael once awed this nation's artists and collectors, and the first half of the show explores the strange intensity of their veneration. The exhibition's second half reexamines carefully the few works by the master in American museums. America now owns 13. Nine are in this show, of which five of the grandest, the "St. George and the Dragon," the "Bindo Altoviti" (once thought a self-portrait), and three superb Madonnas are in the Gallery's own collection.

Yet "Raphael and America," despite these revered paintings, may not please the crowds. The exhibit, which runs through May 8, is not an easily digestible group of pretty pictures. It requires thought and patience. It is a complicated show.

If Raphael's great fame has dimmed in recent years, it is perhaps because his "effortless perfection" galls the modern eye. To those who hunger for the rough, his golden haloed Christs and androgynous Madonnas tend now to look overly sweet, too smooth, too sentimental. But to our early painters, and our richest buyers, too, no painter mattered more.

Copies after Raphael were hung by Thomas Jefferson on the walls of Monticello. John Singleton Copley (1738-1815), the Colonies' best portraitist, thought Raphael the "greatest" painter of them all. Benjamin West (1738-1820), the kindest, most inspiring art teacher of his time, shared that opinion and did his best to imitate the style of the master. Both West and his student, Philadelphia's Charles Willson Peale (1741-1827), chose to name their first-born sons after the Italian. West went even further. When he portrayed his wife and son (the tondo is on view) he did so in a style distinctly Raphaelesque. The deep debt these Americans felt they owed to Raphael is immediately apparent in their copies and pastiches included in the show.

The name of Raphael in those days was synonomous with art--and with the highest sentiments. To every youthful artist his character seemed spotless, his life seemed near perfection. He lived it in deep piety and wholly without struggle, as one could see at once from the saintly ease with which he made his pictures. He never lost, he always won. He took on older masters, Leonardo, Michelangelo, and, despite his joyous sweetness, managed to surpass them. Popes and princes honored him. And Raphael was rich. And wonderfully good looking. At least that was the myth. His willingness to flatter, his greed and ceaseless womanizing were not known in those days. No wonder young Americans took him as a model. And his early death, at the age of 37, lent poignance to his story. Artists who die young--Mozart, for example, or James Dean or John Belushi--attain a special fame.

Raphael's golden life, and his perceived position at the height of the High Renaissance, his much-appreciated sweetness, the smoothness of his flesh tones, and the balanced sinuosities of his compositions, left a deep impression on this nation's art. But the lessons that he taught us were for the most part learned from prints and engravings. Though copies of his paintings hung in many homes, though 19th-century print shops ground out countless reproductions of his famous pictures, nothing he had made himself reached this nation's shores until 1898.

A dozen more have followed. The number might seem small, until one considers how hard they were to get. Never, after all, had he been out of fashion. Kings and queens and popes began collecting Raphaels while he still was alive. That any made it to these shores seems something of a miracle. Two things made it possible. The first was a slight shift in taste. The later, slightly mannerist, paintings by the master had long been highly valued, and most by then were safely lodged in Europe's state collections. But a number of his early works still were in private hands. It was, perhaps, just possible to pry such paintings loose. What was needed was persistence--and huge amounts of cash.

Prices fetched by Raphaels were already awesome; those for other artists were not in the same league. For half a century--from the 1880s until the 1930s--the sums paid out for "Raphaels" set astounding records time and time again.

The first Raphael to come here, his portrait of Tommaso Inghirami, was bought, in 1898, by Isabella Stewart Gardner of Boston, for a price that her advisor, the cunning Bernard Berenson, described as a "mere song." Its price was "almost ridiculous" for one simple reason: It shows a fat and wall-eyed man. Isabella yearned for "a heavenly Madonna." But so did every other collector of her time.

Madonnas cost a fortune. In 1885, London's National Gallery had paid $350,000 for a Raphael Madonna, then the largest sum ever paid out for a picture. J.P. Morgan thought he had a bargain when, in 1909, he paid $200,000 for his Raphael Madonna, a picture that, alas, turned out to be a fake. It was sold off by his son as a "School of Raphael" painting--for $2,500--in 1944.

For authenticated pictures, prices kept on climbing. In 1914, P.A.B. Widener spent $565,000 on the "Small Cowper Madonna," now in the Gallery's collection. That was more than the Metropolitan Museum of Art spent that year on all its acquisitions. "The Large Cowper Madonna," which Andrew Mellon bought in 1928, cost $836,000. Three years later, when the Soviets were short of cash, Mellon was able to acquire his great "Alba Madonna" from the Hermitage in Leningrad for $1.166 million. It was the depth of the Depression. Yet he paid for that picture--as well as for his small, $747,500 "St. George and the Dragon"--in uninflated cash.

Raphael had been popular in America throughout the 19th century, but only in its final years did our great collectors begin to buy in earnest art by the Old Masters of whom Raphael, in those days, was the undisputed king. Brown uses a brief movie, and hand-bound leather catalogues, photographs and clippings to recount in detail the story of these purchases. In the final portion of his show he turns to Raphael himself.

"The Small Cowper Madonna" has been newly cleaned. It is now possible to see that what once appeared to be an indistinct black blob behind the seated Virgin is actually a wall of stone that serves to lock the figures into the sunlit landscape. In addition, an infrared reflectogram, a sort of X-ray of the picture, allows us to examine the astonishingly free drawing underneath the paint.

Brown also demonstrates that the "Portrait of Emilia Pia" from the Baltimore Museum of Art is a surprisingly early Raphael, probably produced before he left for Umbria to study with Perugino. With other X-rays and a sketch, he shows that the cramped and awkward angel in the Metropolitan's "Agony in the Garden" is a late retouching, no doubt made by Raphael at the request of the nuns for whom the work was painted.

Though it was long assumed that the small "St. George" was painted as a present for King Henry VII of England, Brown calls into doubt the evidence for that charming story. But the wall of "St. George" studies that Brown has here assembled (one of these, recently discovered, was just purchased by the Gallery) still is the most revealing sequence in his show.

The exhibit begins by conjuring the "sentimental haze" that was once wrapped around the artist--and then goes on to dispel it. The master was no saint. He died, Vasari tells us, from "amorous excesses." He wasn't even pretty. Attempts to match his exhumed skull with the beautiful young man in his supposed "self-portrait" have proved an utter failure. And it turns out, after all, that making art for Raphael was far from easy work. The artist that we meet here struggled for perfection. He tuned his designs carefully, blending in his pictures hard lessons he had learned from Leonardo, Memling, and others he admired. It was through delicate adjustment--by lengthening a lance or altering a helmet--that he at last achieved that look of perfect ease that earned for him such fame.