The ancient Catholic church in Rome that hides a secret temple of Mithras

In order to find the complex layers of history beneath the church, visitors must enter the current Basilica of Saint Clement, which dates from the twelfth century.

In and of itself, it is a stunning building, which is made in a Romanesque design with thick brick walls and semi-circular arches.

It was intentionally designed to impress the faithful with the power and beauty of the divine.

Every aspect of its construction and decoration is awe-inspiring, even to us, today. 

For example, as you enter the main doors, you are immediately greeted by a spacious nave, with rows of sturdy columns, and a high, coffered ceiling.

A keen observer will see that these columns were reused from earlier Roman buildings during the medieval period.

Perhaps the most striking image of the current structure is the shimmering mosaic of the apse which depicts a golden mosaic crucifixion scene, where Christ appears on a blue cross surrounded by a series of entwined vines, which is meant to call to mind the representation the tree of life.

Around the central composition, there are figures of saints, including Saint Clement, while the lower section of the mosaic depicts sheep emerging from the cities of Bethlehem and Jerusalem. 

Elsewhere in the basilica, other elaborate frescoes depict the dramatic lives of the saints Clement, Ignatius, Servulus, and Ambrose, while impressive Cosmatesque patterned marble floors boast colorful stone tesserae that form interlacing designs.

The baldachin above the altar features finely carved columns and an ornately decorated canopy.

In the confessio below the altar, sacred relics are housed. However, there is a much older history to this church than meets the eye.  

One of the most striking features of the basilica is a spectacular marble choir enclosure that was specifically imported from Constantinople in the sixth century.

It is made from Proconnesian marble from the island of Marmara and features finely carved panels with intricate geometric patterns and stylized foliage.

It forms a rectangular barrier around the clergy’s seating area, as a sacred boundary that would have separated the presbytery from the laity during liturgical ceremonies.

However, to the right of the entrance, there is a set of relatively recent 19th century steps that leads you down into an even older structure. 

Before the current basilica was built in the twelfth-century basilica, on this site stood an earlier church, which was an important place of Christian worship during the fourth century CE.

This one was built within a century after the legalization of Christianity under Constantine, which meant that it is one of the city’s first significant places of Christian assembly.

This ancient basilica featured a wide nave with side aisles, which was designed to accommodate large number of Christians who flocked here for religious ceremonies.

In fact, the fragments of marble columns and sections of mosaic flooring still survive. 

There are precious frescoes from this older church which miraculously still cover sections of the walls and preserve some of the earliest known medieval Christian art in Rome.

Among these paintings is a striking depiction of Saint Clement himself, who was an early bishop of Rome.

Another fresco is of particular interest to church historians, because it depicts a miracle story, which shows an episode where a child was rescued from drowning in the Sea of Azov through divine intervention.

Nearby, another piece known as the Sisinius Fresco is famously accompanied by an 11th century example of written vernacular Italian: one of the earliest historical inscriptions in Italian. 

The most important location in this layer, especially to many pilgrims to the basilica, is the tomb of St. Cyril, a Byzantine missionary who introduced the Glagolitic script to the Slavic peoples during the 9th century.

After completing his mission to Moravia alongside his brother Methodius, Cyril had traveled to Rome in 869, where he sought papal approval for the use of the Slavic liturgy.

Sadly, soon after his arrival, he fell ill and died on February 14, 869, aged just 42 years old.

He was then buried in the original basilica of Saint Clement in a marble tomb. As a result, over time, the site became a place of pilgrimage, attracting worshippers from Eastern Europe who revered him as the apostle of the Slavs. 

Over the centuries, this oldest church underwent a series of structural modifications as the Christian community in Rome grew.

By the ninth century CE, there is evidence that sections of the church’s walls were further strengthened to maintain the integrity of the building. 

Several hundred years later, the church was deemed unsafe, and a new one was planned to replace it.

As such, the twelfth-century basilica was constructed above it and much of the earlier structure was filled in and buried to become its foundation.

However, this process preserved its frescoes and architectural details and, when it was rediscovered in the nineteenth century, it revealed a long-lost chapter of Christian history.

In 1857, Father Joseph Mullooly began excavating underneath the church and what he found there surprised everyone. 

Visitors can find another set of stairs, built by the Romans themselves in the 1st and 2nd centuries AD, that takes people down further, beneath the church ruins.

Here, they can find the remains of two buildings that would have been located in the valley between the Celio and Esquiline hills.

One was a private house (known as a domus), which contained a series of rooms and passageways.

Next to it was a narrow alleyway or street, just 50cm wide, that was used by the people of Rome during the time of the emperors.

The second building had exterior walls made of large, tufo blocks. It is still not certain what it was used for, but the most common assumption is that it was part of the imperial mint, which produced the coins bearing the emperor’s image.

Today, these ruins provide structural support for the two layers of church above them.  

The domus was a particularly well-appointed Roman house, which was constructed some time during the first century CE, probably during the reign of Emperor Vespasian.

The building suggests that the owners had significant wealth and social status, as interior walls were decorated with painted plaster of a style that was popular among elite households in the early imperial period.

This suggested that the residents had maintained a very comfortable standard of living for the age.  

By the second century CE, sections of this house were starting to be repurposed.

Some doorways had been sealed, either as a way to stabilize the buildings or to use these rooms for more practical purposes, such as storage.

Then, by the fourth century, the structure was largely abandoned. Ultimately, the construction of the fourth century Christian basilica filled in and sealed off much of the building, which preserved it.

The walls of the old structures remained buried beneath centuries of construction, hidden from view until archaeologists uncovered them in the nineteenth century.

And this leads us to the astonishing secret that was hidden within the oldest layers of the ruins. 

In at least three of the rooms of the ancient Roman house, there was hidden a temple devoted to Mithras, known as a Mithraeum.

The cult of Mithras gained popularity particularly among men who valued its emphasis on discipline and loyalty.

At this point in Roman history, the Mithraic cult was growing in popularity among soldiers, merchants, and even civil administrators in the empire.

It appears that one of the rooms of the ancient Roman house had been renovated in order to transform it into a sacred sanctuary at the end of the second century CE.

Even when it was created, it was a dimly lit sanctuary that was intended to only be seen by a select few.  

As part of the secret rites of this religion, worshippers were to enter a dark chamber, designed to resemble a cave, and position themselves on either side of a central aisle.

The walls were flanked by raised benches where initiates would be expected to participate in a series of sacred ceremonies, including a communal meal.

At the far end of this chamber was a sculpted relief depicted Mithras slaying the cosmic bull (known as the Tauroctonia), which was the most important central image in the cult’s belief system.

It was thought that the mystical slaying of the animal was believed to bring spiritual renewal and cosmic balance to the followers of Mithras. 

During the second century CE, Roman religious was undergoing a series of changes.

The traditional polytheistic practices and the imperial cult were beginning to fall out of favor, and there was a shift towards more exclusive and secretive practices, such as those of Mithraism.

This religious evolution meant that the state-sanctioned temples did not hold the same appeal as Mithraic worship and other mystery religions, which involved secretive initiation rites, a strict hierarchy, and ceremonies that were intentionally conducted away from the public eye. 

In the early fourth century CE, the Mithraeum was still in use. It was only after Emperor Constantine legalized Christianity through the Edict of Milan in 313, Christian communities began constructing formal places of worship across the city.

As a result, Mithraic sites gradually fell into disuse. Some were abandoned, while others were repurposed for new religious structures.

The Mithraeum beneath the Basilica of Saint Clement remained sealed beneath layers of construction. 

If you would like to learn more about this fascinating church, or want to visit for yourself, you can find more information on the basilica's website.