Few places better exemplify the nature of Rome as a city of many layers than the church of San Clemente. Under the 12th century church, at a depth of six metres (twenty feet), sits a 4th century church. Below this, at a depth of about nine metres (thirty feet), are buildings from both Republican and Imperial Rome. To visit San Clemente is to travel back in time over 2,000 years.
San Clemente was built during the reign of Pope Paschal II (r. 1099-1118), but, for centuries, it was thought to be the church to which St Jerome referred in 392. The existence of a second church, buried under the first, went completely unsuspected until 1857 when a priest and amateur archaeologist, Father Joseph Mullooly O.P., made the discovery of his life. Father Mullooly dug through more than twenty feet of rubble to reach an earlier church, sitting almost directly beneath the present one. This was not an end to his discoveries, for below the newly excavated church, he later came across part of an insula (an ancient Roman apartment block) from the Republican era. Subsequent excavations by other archaeologists revealed a house and a mithraeum, both from the Imperial era.
San Clemente is one of the few medieval churches in Rome to have retained its atrium or courtyard, which is entered through a 12th century portal, known as a prothyrum. On entering the church one’s eye is drawn immediately to the stunning mosaics which decorate the apse, the most beautiful to be seen anywhere in Rome.
The nave is divided from the side aisles by a collection of ancient granite and marble columns. The pavement is laid out in a style known as Cosmatesque, after the Cosmati family, workers of marble, who flourished during the 12th and 13th centuries. The style, also known as opus alexandrinum, was used most extensively for church floors, but also for pulpits, columns and episcopal thrones. The Cosmati used stones salvaged from the ruins of ancient Roman buildings to create their geometric designs.
Discs of porphyry and granite lead to the schola cantorum, the 6th century choir enclosure, which was rescued from the older church. It is decorated with the monogram of Pope John II (r. 533-55). The ambo on the left side and the Paschal candlestick were later additions. A marble screen separates the main body of the church from the high altar, where we find the tombs of St Clement, to whom the church is dedicated, and St Ignatius of Antioch. There is an image of an anchor (the attribute of St Clement) entwined with a cross on the canopy over the altar. At the back of the apse is an ancient throne. The bottom of the apse wall is decorated with large 14th century frescoes of Christ, the Virgin Mary and eleven of the apostles (St Matthias is missing). However, it is the beautiful 12th century mosaics that are the glory of the church.
The second great attraction of San Clemente is the Cappella Castiglione, which can be found at the beginning of the left aisle.
By the advent of the 18th century, San Clemente was in a very poor state of repair. It was restored at the behest of Pope Clement XI (r. 1700–1721). The splendid wooden ceiling, which sports the pope's coat of arms, was one of the products of this restoration. The central fresco depicts the Apotheosis of St Clement, the work of Giuseppe Chiari (1654-1727). The frescoes on the upper walls of the nave depict scenes from the lives of St Clement (left wall) and St Ignatius of Antioch (right wall).
The original 4th century basilica, and the ancient Roman remains, can all be visited. The first basilica was slightly larger than the later one, which basically replicated its design. A few frescoes from as early as the 9th century have survived.
Underneath the 4th century basilica are the remains of two buildings, which are separated from each other by a narrow passageway. On one side of the passage is a brick insula (an ancient Roman apartment block), while on the other side are the ground-floor rooms of a mansion from the 1st century CE. The latter may have belonged to Titus Flavius Clemens, who is thought to have been executed in 95. His house might have been the venue for gatherings of Christians until the church was built in the 4th century. Clemens is not to be confused with his contemporary Pope St Clement I, to whom the church is dedicated.
Across the passageway are the remains of a small Mithraic temple (late 2nd or early 3rd century). It consists of an ante-chamber with stone seating, a triclinium (dining-room) with a marble altar and a third chamber, which may have functioned as a room for the instruction of neophytes. The seven niches in the chamber might represent the seven stages of the initiation into Mithraism. Mithraism was a mystery religion, which was practised across the Roman Empire from the 1st to the 4th century CE. Membership was restricted to men and the religion was particularly popular with soldiers.