During the 1960's and 1970's, I spent a lot of time exploring the medieval churches and cathedrals of central and southern France, and I soon came to expect to find, in some corner or other, a sculptured relief of a scallop shell, the symbol of St. James the Apostle. The shells were there, a bit of reading revealed, because the buildings had served in the 11th and 12th centuries as hostels for the million or more pilgrims who each year walked from France - and from Britain, Germany and Scandinavia - to the great shrine at Santiago de Compostela in northwestern Spain, where the saint's bones were said to have been unearthed in the ninth century.

They wore a kind of pilgrim's uniform of cape, sandals and broadbrimmed hat festooned with scallop shells; they carried staffs with gourds attached to hold water, sometimes spending a year en route, living in fear of robbers and brigands. For a time, the greatest Christian shrines in the world were Rome, Jerusalem and Santiago, which means St. James in Spanish. Not by chance did Pope John Paul II choose this venerable meeting-place at the edge of the continent to appeal last year to ''Old Europe'' to ''find yourself, be yourself, discover your origins.''

It is still possible to follow the route from one of the main gathering points in France, such as the Tour St. Jacques in Paris, between the Bastille and the Louvre. (St. Jacques is the Apostle's name in French; memories of the great pilgrimages survive not only in the name of the tower but also in the French name for scallops, which are called coquilles St. Jacques).

The Way of St. James, as the route is called, divides and reunites several times as it winds from northern France to the southwest, passing Autun and Cluny, Souillac and Conques. It crosses the Pyrenees at Somport or at Roncesvalles, where in 778 the Basques massacred the rear guard of Charlemagne's army under Roland - an episode glorified in France's first epic poem. Then the way leads across northern Spain, through one of the most glorious treasure houses of Romanesque architecture, toward Galicia. All along the Spanish part of the route, the direction signs, each decorated with a scallop shell, give the distance to Santiago for modern pilgrims.

For one reason and another, I had never made it to Santiago, despite good intentions. But last summer I happened across a pair of newspaper articles that goaded me into action. One, in a French newspaper, reported that 1982 would be an ano santo, a holy year, because St. James's Day, July 25, fell on a Sunday. This, the piece said, happened only once every four to 11 years, or some such thing, and would bring tens of thousands to the city. The other article, a profile of Sir Sacheverell Sitwell, the writer and esthete, in The Times of London, quoted him as saying that ''there is nothing finer in Europe than Santiago de Compostela,'' including Venice and Florence.

Not even Sir Sacheverell's enthusiasm prepared my wife and me for the spectacle that greeted us as we drove into the center of the city at midnight, 48 hours before the big day, after an exhausting if exhilarating journey over barely adequate roads. We had babbled excitedly after visits to the monastery at Silos, the Romanesque church at Fromista, the Royal Pantheon and the tiny Mozarabic church at Leon, and the pre-Romanesque buildings on a grassy hillside above Oviedo, but we were stunned into unwonted silence by Santiago de Compostela.

The center of the city is a vast square, the Plaza de Espana, which is longer than a football field. Because we had been lucky enough to get a room at the Hostal de los Reyes Catolicos, built from 1501 to 1511 by Ferdinand and Isabella and now a sumptuous hotel, we were permitted to drive right into the square and park. As we got out of the car, we could hear young voices somewhere nearby singing hymns; above us towered the softly floodlit Obradoiro facade of the cathedral, a Baroque masterpiece by Fernando Casas y Novoa, built in 1750 to protect the 12th-century west front.

T he light dramatized the already dramatic - the swirls and curves of the ornamentation, the slender towers, the quadruple flight of rectilinear steps leading up to the door. Splendid buildings, three hundred, four hundred, five hundred years old, stood on other sides of the plaza. George Henry Borrow, the 19th-century British traveler, had got it exactly right when he said Santiago was a place ''in every respect calculated to excite awe and admiration.''

The cathedral is, of course, the focal point of the celebration, which takes place every year, although not ordinarily on quite the scale of the holy years. Pilgrims still begin their visit by approaching the Door of Glory, a 12th-century work by a carver known only as Master Mateo, which ranks as one of the half-dozen greatest pieces of Romanesque sculpture. It remains much as it was 800 years ago, a vast composition of three portals opening into the cathedral, embellished with thousands upon thousands of portraits in stone. The eye travels at once to the central opening, which is bisected by a pillar bearing a statue of St. James, gazing with the utmost serenity at some far point, and crowned by a tympanum showing Christ the King engulfed by angels. Millions of the faithful, touching down the austere nave to the richly ornamented sanctuary, which seems intolerably gaudy after the serene simplicity of the Door of Glory and the exuberant equilibrium of the facade, to kiss the cloak draped over a 13th-century statue of the saint. Silver and precious stones, incredibly elaborate carving - and yet none of the spirituality, for me at least, that resides in the smallest and least of Master Mateo's details.