RIYADH, Saudi Arabia, Feb. 8 - At a secret location every Sunday evening, a young Catholic priest does a dangerous thing. He says Mass.
He arrives in street clothes and retrieves his vestments, liturgies, hymnals, Bibles, crucifix and chalice from a locked cupboard. Discretion is crucial, he says, because the Mutawwain, the street-patrolling morality police employed by the kingdom, has threatened to hunt him down.
Bearing witness to the faith takes on special meaning in this royal theocracy. Here, Islam is the only official religion and all citizens must be Muslims. The Constitution consists of the Koran and the teachings of the Prophet Muhammad. Proselytizing is punishable by a prison sentence.
Conversion by a Muslim to another religion is considered apostasy, punishable by death if the accused does not recant. Non-Muslim worshipers who engage in overt religious activity that attracts official attention risk arrest, lashing and deportation.
In describing Saudi Arabia in its 2001 report on religious freedom, the State Department was unusually blunt: "Freedom of religion," it stated, "does not exist."
The situation is particularly painful for American troops. They are offered a range of religious services, with the help of military chaplains.
But they must worship in private, even though many of them are protecting the kingdom from outside threats. And soldiers who wear a cross or a Star of David must keep the symbols hidden.
"We have all these fine young American men and women over here," one chaplain said. "They're great Americans. They're great soldiers. Yet they're expected to surrender their religious practices when they arrive."
President Bush has called Islam a great religion and described the American people as both religious and tolerant. When he addressed a joint session of Congress shortly after Sept. 11, he said that the "barbarians" who attacked the United States "hate our freedoms, our freedom of religion, our freedom of speech, our freedom to vote and assemble and disagree with each other."
But neither Mr. Bush nor any of his national security advisers have criticized the refusal of Saudi Arabia to allow Americans and other foreigners to worship freely. The United States, like other governments, has agreed to a compact dictated by the Saudis: if you have to practice your religion, do it in secret.
On one recent Sunday evening, about 200 Catholics from at least two dozen countries gathered to worship. Among them were diplomats, corporate employees, drivers, domestic help, spouses and children.
One of the organizers checked names against a master list before allowing them into a reception room. There, a makeshift altar was set with a crucifix, candles and baskets of flowers. Outside, regular Saudi police officers stood watch, apparently to protect the service from being disrupted by the less disciplined morals police, but also perhaps to note the identity of worshipers.
The worshipers recited the liturgy, sang hymns and received Communion. Someone had taken away the chairs that day, so they stood through the hourlong service and knelt on the wooden floor.
"Tonight, you are truly standing up for the faith," the priest told his flock. Quoting the words of Jesus Christ in the Gospel according to St. Matthew, he added, "Where two or three are gathered together in my name, I am in the midst of them."
The kingdom bans non-Muslim clergy from entering the country to conduct religious services, although some are allowed in for other reasons. There are few resident priests and ministers, and worshipers often conduct their own services. Protestant, Mormon and Jewish services are also held in secret.
Non-Muslim services are risky enough that worshipers never talk about them on the telephone. The State Department freedom of religion report states that there is no clear definition of "private worship" and cites instances of "arbitrary enforcement" of the Islam-only rule.
"I call it the `catacomb church,' " said one senior diplomat, a Catholic.
"When there are inquiries about what we are doing," he added, "we say, `The ambassador is organizing a meeting for nationals.' "
The uncertainty over what is private was underscored in Jidda last summer, when an Indian businessman who was leaving the country rented a hall andthrew a going-away party, according to foreign diplomats there. When the 800 or so guests arrived, a church service began.
The Saudi authorities ignored it. But when he did it a second time, they arrested him and 15 others. At least half of those arrested were deported.
"It was the biggest church service Jidda had ever seen," one diplomat said. "The rule is that if it's discreet and small it's O.K. But 800 was way off the scale."
Islam was founded in the seventh century by the Prophet Muhammad, who, Muslims believe, received God's revelations through the archangel Gabriel and recorded them in what became the Koran. Moses and Jesus are among the prophets who are particularly honored in the Koran, which also recognizes the Torah, the Psalms of David and the Gospels as God's revelation.
Although the practice of Christianity and Judaism is allowed in many Arab and Muslim countries, that has not been the case in the Saudi kingdom, which is home to Mecca and Medina, the two holiest shrines in Islam.
For years, the Saudi-born terrorist Osama bin Laden has vowed to purge the kingdom of the "infidel" American military presence from the country of the two holy shrines. Those threats have made American military commanders particularly uneasy about discussing religious services for their troops.
But not the soldiers themselves. "I have three Bibles," said one young M.P. at an American military installation, who said he was able to attend Protestant services. "Just let someone try to take them away."
In an interview two weeks ago, Crown Prince Abdullah bin Abdul Aziz, Saudi Arabia's de facto ruler, was asked by an American reporter whether his kingdom had an image problem among Americans.
Americans did not understand the absence of elections, limitations on women's rights and the lack of tolerance for other religions, the reporter said, adding that during a maiden visit to Saudi Arabia on Easter Sunday nearly two decades ago, she had been unable to attend Mass.
Crown Prince Abdullah replied that the presence of the two holy shrines on Saudi soil were the "primary restrictions" in making changes in the kingdom. "Our faith and our culture are what drive the country," he said.