A prayer service was held Saturday evening for Christians around the world who are being persecuted for their faith. The service took place at the Basilica of the National Shrine of the Immaculate Conception in Washington, D.C.—not in the huge upper church, which seats thousands of people, but in the much smaller Crypt Church downstairs.

The setting was appropriate. For some people, the very word “crypt” has creepy connotations: a dark, hidden place underground, often containing the bones of those long dead.

In addition, “crypto” has been used to denote things that are hidden, and “Crypto Christians” is a term that has gained currency, referring to followers of Christ who must for one reason or another hide their faith from all but fellow believers.

The group that held Saturday’s prayer service, Aid to the Church in Need, sprang up in the wake of the Second World War to assist refugees in Europe. Their work has expanded to assist “crypto Christians” around the world.

But the concept of crypto-Christianity has a long history. One might almost say it was there at the very beginning, as Jesus often told his disciples and those he healed to keep secret the miracles he had performed. Until Pentecost, the Apostles were lying low, fearful that they too could suffer the same fate as Jesus.

As historian Philip Jenkins pointed out, it’s not easy to study Crypto Christians because the very nature of their existence is one that is hidden. “In theory, hidden believers should be immune to study, as they would never break cover; the people who can be studied are only the less discreet,” he wrote in an article in the Christian Century.

But we can be pretty sure that somewhere in the world, they continue to exist today, in countries dominated by atheistic regimes or religions hostile to Christianity.

Here is a brief overview of the history of crypto-Christianity:

Roman Empire. Alan Schreck, author of The Compact History of the Catholic Church, says that the Roman government at first ignored the Christians: they were mostly from the lower social classes and included numerous women, common people and slaves. But in A.D. 64, Emperor Nero blamed them for a great fire in Rome. “Christians buried their dead, prayed and sometimes hid in catacombs (underground burial vaults) of Rome during these early persecutions,” he writes.

The word catacomb would be applied again in the future to crypto-Christians, even if their places of worship were not underground burial vaults.

France. In the wake of the French Revolution, some 30,000 or 40,000 priests were driven into hiding or exile after they refused to take an oath to the government that would compromise their loyalty to the pope. Many were later killed when the French revolution became more radical in 1793, Schreck says. The rulers tried to push Christianity out of society, favoring a “religion of reason” instead.

“The true faith of the people could not be suppressed, however,” Shreck writes.

Japan. St. Francis Xavier arrived in Japan on August 15, 1549, and had much success initially in spreading the Gospel. Within 50 years there were about 300,000 Christians in Japan. Things began to change for the worse when a vessel carrying Spanish Franciscans ran aground near Tosa, and the captain strangely remarked that the missionaries had been sent to prepare for the conquest of the country. The Japanese ruler gave orders to draw up a list of Japanese Christians, and on February 5, 1597, 26 were crucified at Nagasaki. Persecution subsided for a while, but recommenced in 1614, when shogun Ieyasu Tokugawa decreed that Catholicism be abolished.

In order to root out hidden Christians, officials required people to trample on an image of Christ—a fumi-e. Kakure Kirishitan (“Hidden Christians”) worshiped in remote places, and in some cases masked their faith by practicing the dominant religion. In at least one Buddhist temple today can be found a small feminine statue next to a larger one of Buddha. When Christians bowed before the smaller statue, they were praying to the Blessed Virgin Mary.

Ottoman Empire. Though Islam spread rapidly from its founding in the 7th century, it wasn’t until the 14th that persecution of Christians in the Near East become systematic and violent.

“Matters changed swiftly during World War I,” Philip Jenkins writes in Christianity Today. “Massacres and expulsions all but removed the once very large Armenian and Greek communities in Anatolia (now Turkey). Counting Armenians, Assyrians, and Greeks together, murder and starvation killed more than two million Christians between 1915 and 1922.” Jenkins writes:

The Church of the East, the ancestor of the Assyrians and Chaldeans, perfectly illustrates that long survival—and profound current crisis. The disasters of the 14th century reduced that once transcontinental body to a much smaller remnant. That vestige continued within Iraq, Syria, and Anatolia for seven centuries. Throughout that latter period, hard-line Muslim jurists and demagogues competed to invent new humiliations to inflict on Christians: limits on what those believers could wear, the houses they could own, and the horses they could ride. At the worst of times, Christians wore rags to avoid giving any impression of wealth, which invited others to take their property.

Soviet Union. As one way of defusing Ukrainian nationalism, Soviet leader Joseph Stalin orchestrated a Church council that effectively banned the Ukrainian Greek-Catholic Church in 1946. The faithful continued to practice underground, and bishops and priests went to prison when they were caught.

Bishop Vasyl Velychkovsky discretely welcomed underground Christians to his apartment in Lviv, Ukraine, quietly celebrating Divine Liturgy and even ordaining priests who in turn would minister in secret. For an altar and liturgical vessels, he used a simple table and household items that could be scattered around the apartment in case the police paid a visit.

“Even when institutional churches vanish, believers persist in many different forms,” historian Jenkins wrote. “As Anatoly Lunacharsky, the frustrated Soviet minister of education, complained in 1928, ‘Religion is like a nail: The harder you hit it, the deeper it goes into the wood.’ Sometimes it goes in so deep, you can’t even see it.”

Writing in 2014, when the Islamic State group was making headlines for its murderous campaign against Christians, Yazidis and Muslims deemed less than faithful, Jenkins predicted that in Iraq and Syria the practice of Christianity would be “much more difficult under an extreme Islamist regime than under the secular Ba’athists, but ‘cryptos’ have often endured for astonishingly long periods, until gentler times return.

“Shall we talk about the extinction of Middle Eastern Christianity?” Jenkins concluded. “Come back in 500 years. We’ll see then.”