IF YOU JUDGE by the pictures, the Makkah Hilton is a nice place to stay. There's just one catch, as the Web site notes. The five-star hotel "is exclusively sited within the Holy City which, by national and religious law, is only accessible to visitors of the Muslim Religion."

This law is something of a singularity among major religions, because it isn't merely the Grand Mosque that is off-limits to nonbelievers, the way, for instance, a Mormon Temple is. It's a city--a major city with hotels, supermarkets, schools and a population of 1.2 million people. (The city of Medina, population 700,000, also forbids non-Muslims.)

What are the roots of this apartheid? The Koranic revelations were given to the prophet Muhammed in Mecca, which was then a pagan place. Soon after, he left Mecca and traveled to Medina, where he assembled an army, returning to conquer Mecca in A.D. 630. "The Prophet then ordered, on the basis of what he said was God's command to him, that the environs around Mecca should only be for Muslims," explains Seyyed Hossein Nasr, a professor of Islamic studies at George Washington University.

The custodians of Islam take the ban seriously, and they have constructed a large apparatus to keep infidels out. In "The Saudis," Sandra Mackey's account of living in Saudi Arabia several years ago, she recalls trying to drive near Mecca (with her husband at the wheel, of course): "Billboard-size blue and white signs in both Arabic and English appeared along the road, warning non-Moslems to turn back." She saw religious authorities and Saudi policemen "lounging in a small wooden building adjacent to the road." Eventually, "we were forced off the road by one of the angry policemen." She was fined about $100 and turned away. (What's the penalty for actually being caught inside Mecca? The Saudi embassy refused to return calls.)

Ali Al-Ahmed, executive director of the Washington-based Saudi Institute, explains that these posts "check your religion, basically." He notes that, "if you're a Saudi, of course, there is no problem. But if you aren't, your ID says what your religion is." If you're wondering why it's not a problem if you're a Saudi, Ms. Mackey explains it best by quoting a passage from a Saudi hotel directory: "Islam is the official religion of Saudi Arabia. Churches of other religious denominations do not exist in the kingdom."

Professor Nasr has a more benign view. When traveling to Mecca, drivers are stopped at a toll station, he explains (the city has no airport): "Somebody comes forward and looks and says, 'Are you all Muslims?' And the people will say 'yes' and they'll say, 'Go on.'" But "if the authorities become suspicious because someone doesn't look like a Muslim, they'll say, 'Recite the first chapter of the Koran' or some such thing which all Muslims know by heart."

The ban sometimes creates logistical woes. Companies that rely on skilled workers often resort to using auxiliary offices outside the city. Mackey tells of the building of a hotel designed by a Western architect. The Saudis refused to allow him into the city and, she writes, "insisted that he stand on a hill outside of town and direct the work through a telescope."

Curious, daring souls have, of course, run the blockade for a long time. In the 19th century, a handful of Orientalists disguised themselves as dervishes and made their way inside, the most notable being John Lewis Burckhardt and Sir Richard Burton, who wrote of their experiences. (In 1917, T.E. Lawrence snuck in for the sole purpose, it seems, of buying a gold dagger from a particular merchant.)

Even today, there are ways around the barrier. Occasionally nonbelievers will surreptitiously enter the city in the company of Muslim friends. One Muslim confides: "I have some Christian friends who have visited Mecca in the last few years--also a Jewish friend."

Officially, the U.S. is unfazed by this state-sponsored segregation. A State Department official tells me that, while "Saudi Arabia is a candidate for designation as a 'country of particular concern' under the International Religious Freedom Act, . . . the most acute problems that non-Muslims in Saudi Arabia face concern the almost virtual prohibition on engaging in their own worship. We are not aware of many demands by non-Muslims to visit Islamic holy sites."

But even at the State Department, the ban rankles. "Every religion needs to have its own freedom of assembly and be able to protect the integrity of its holy places," said one State Department expert. "But it's just absurd that it goes to these great lengths. . . . Even if you don't care about lofty ideals like religious freedom or openness, we're starting to see some real connections between religious intolerance and terrorism."

In 1995, incidentally, Saudi money funded the building of an enormous, $50 million mosque in Rome, just a stone's throw from St. Peter's Square.