THE GREAT CATHEDRAL is packed with tourists, worshipers, and day-trippers stopping for a bit of culture on their way to dinner. They crowd into the sanctuary of Santiago de Compostela to glimpse great art and architecture, or to venerate the relics of St. James the Great, apostle and martyr, a traveling evange-list whose remains are said to reside in a silver casket beneath the altar.

Where the two great wings of the cathedral form a cross, a priest prepares the altar for Mass. And to one side, kneeling on wooden benches, are the “real” pilgrims. They're not dressed for church. Their pews are clogged with backpacks and walking sticks, and their bare knees are brown against the wood. Somewhere about each of them, tied to a camera bag or bedroll, is a scallop shell—traditional symbol of this particular pilgrimage, and an unofficial passport to the hospitality of the residents along the way.

Even as the crowd murmurs around them, the 50-or-so peregrinossit silently, a little island of quiet amidst the echoes in the great stone edifice. They are athletes of faith, the searching souls who put feet under their convictions and walked, or bicycled, across hundreds of miles of mountains and highways to get to this holy place. Those who travel on horseback, bike, or foot for at least 90 miles along the ancient pathways, and can prove it with a series of stamps on an official pilgrimage “passport,” are shown special honor at the cathedral. They're given an elaborate certificate, called a compostela, and a simple dinner at the local hostel. And at a special noon “Pilgrim's Mass,” a cathedral functionary reads off the day's statistics. “Fourteen pilgrims from Australia. Six from Holland. United States: 12,” the voice rings out. Eyes fill with tears—some promise fulfilled, perhaps, or simple relief at the end of a long journey. Or perhaps a little mourning that the adventure is ended. Finally, the priest intones “The Mass is ended. Go in peace.”

The travelers quietly embrace one another, shake hands, exchange kisses. Most were strangers many towns back, but now will correspond and relive their pilgrimages in memories of a lifetime. The Camino changes hearts. There's something mystical along that trail, some power left behind by the thousands—the popes and paupers, saints and sinners—who walked those stones before.

Pilgrims have made their way across Spain to Santiago de Compostela for more than a thousand years. Several paths crisscross Europe, Spain, and Portugal, funneling travelers to the upper-right corner of Spain. The city still has plenty to offer vacationers. Here are 500-year-old buildings decked with gargoyles and layers of symbolism. Ancient psalms are chanted daily at a long list of convents and monasteries. Silversmiths and jewelers have a plaza of their own, and the ancient walled city is packed with tiny shops and fragrant cafes.

Legend says the Apostle James traveled to Spain to preach the Gospel, and his followers buried his body there after his martyrdom. But the saint's grave was forgotten among the cemetery remains that grew up around it, and as paganism spread across the land. But a legend sprang up there, about 700 years later. A holy hermit supposedly saw bright stars shining over a weed-covered hillside. He followed the light, and uncovered the long-neglected sepulcher.

The tale took hold, and the first tiny church was built on the site by 813 A.D. Acity soon grew up, and pilgrims were drawn by rumors of miracles. By the 11th and 12th centuries, the pilgrimage to Compostela was famous throughout Europe. A “modern” cathedral was begun in 1075, the heart of which is still part of today's sprawling stone building.

The major pilgrims’ road to Santiago, known as el Camino, carried millions of visitors across northern Spain—and they brought along their folklore, culture, religion, science, and art. Monasteries, churches, hospitals, and resting-places all grew up along the pilgrims'pathway. Many of the original roadside rests are gone, but many that remain are now spectacular hotels, cozy, time-worn churches, or echoing, empty museums of boom-times past.

The way has always been long, and often dangerous. Over the years, travelers faced bandits, disease, wolves, wild boars, and crooked innkeepers. Criminals were sent on a Compostela pilgrimage to atone for their sins. Other adventurers went in place of wealthy sponsors, or to represent an entire village before the saint.

Many active 20th-century tourists fly across oceans and rendezvous with trip out-fitters to follow the long trail. Some come out of love for history, culture, scenery, or religious fervor. It's a fantastic challenge for hikers or mountain bikers. Food and lodgings are cheap and easily found, and range from five-star former palaces, to cramped bunk beds in state-run pilgrim hostels.

As the miles grind on, each traveler finds his own pace—and something spiritual happens. The Camino winds through medieval villages, alongside interstate highways, past garbage heaps and industrial back yards. It twists over mountain vistas, and sloshes past wet meadows that rumble and bark in the language of frogs. The ancient stones disappear under mud or rose petals, cow dung or pavement. St. Francis of Assisi walked here. Pope John Paul II made the trip. Francis Xavier, Teresa of Avila, and a host of lesser-known holy people viewed these stone fences and green fields, palaces and wayside chapels.

The mind settles into the steady pace of pedal or footfall. The distractions of life at home fall away. Eventually, the pilgrims say, the roadside blooms with metaphor and revelation.

Vickie Ward, an outdoor vacation outfitter from Seattle, said she has seen the metamorphosis dozens of times, in hundreds of travelers she has shepherded on her “Camino Tours” biking and hiking trips to Santiago.

“I'll have someone show up all fired-up about a great bike trip, a fun vacation, ignorant of anything historical or religious, not really caring anything about it. But by the time they're finished, they're totally nuked by the experience,” she said. “They're talking about their ‘journey,'and all the personal discoveries they've made. Even me. I was never religious, and now I'm an active member of a Church. I found my faith along this trail. And every time I do the trip, I am moved. I find some new treasure, some new truth.”

Mara Lyons, a bicyclist from Berkshire, England, described her spiritual eye-opening. Late in April, just after their trip began, an elderly lady in a rural town invited Lyons and her husband, Len, into her home for a snack of bread and wine.

“We could tell she was very poor, and she really couldn't afford to be entertaining us,” Mara Lyons said. “She wouldn't accept any money—she was almost insulted. She just told me, when I left, ‘pray for me, peregrina, when you get to Santiago.'

“I thought it was a little silly at the time, but I told her I would—it made her happy. So I carried that promise with me the entire trip. I felt responsible for keeping it. It turned my mind toward my duty to others, how I'd grown away from simple kindness. It gave the entire trip a spiritual edge. I've never been on a more meaningful holiday. And arriving in Santiago? It's a stunning, beautiful, ancient city. But beyond that, I had a purpose to be here. I had a prayer to make.”

Rebekah Scott is religion editor atThe Blade, a newspaper in Toledo, Ohio. She made the Camino pilgrimage, on foot, in May.