S.Clemente is one of the eighteen titular churches in Rome known to have existed as early as the 3rd century A.D. It all started with a roman house owned by Roman consul and martyr: Titus Flavius Clemens, who was one of the first among the Roman senatorial class to convert to Christianity. Titus Flavius allowed his house to be used as a secret gathering place for fellow Christians, the religion being outlawed at the time. This man was sentenced to death during one of the very first christian persecution under emperor Domitian. Much later under Pope Siricius (384-399) a basilica was erected over the house of the senator and other roman dwelling and was dedicated to St Clement, the third roman bishop after St. Peter. In the early Christian romance or novel known as the Clementine literature, Titus Flavius Clemens is identified with Pope Clement I - fourth Bishop of Rome, saint and martyr - an identitification which has no extant basis in actual historical fact. However, the Pope may have been a freedman of the consul. The small church underwent expansion, acquiring the adjoining insula and other nearby pagan buildings; Architects began work on the complex of rooms and courtyards, building a central nave over the early church site, and an apse over the former Mithraeum.
The church we see today where mass takes place daily was rebuilt after the original church was burned to the ground during the Norman sack of the city under Robert Guiscard in 1084. Abandoned and set underground ever since was uncovered by an irish father only in the XIX Century. The remains can now be seen below the current church. Apart from those in Santa Maria Antiqua, the largest collection of early medieval wall paintings are to be found in the lower basilica of San Clemente. Nonetheless today, the upper basilica of St. Clemente, is one of the most richly adorned churches in Rome. The inner space is divided in three naves with arcades made of ancient marble or granite columns. The 12th-century schola cantorum , space for chanting, incorporates marble elements from the original basilica . The episcopal seat stands in the apse, which is covered with mosaics on the theme of the Triumph of the Cross that are a high point of Roman 12th century mosaics. Worthwhile is also the stunning Cosmatesque inlaid floor. Irish Dominicans have been the caretakers of San Clemente since 1667, when England outlawed the Irish Catholic Church and expelled the entire clergy. Pope Urban VIII gave them refuge at San Clemente, where they have remained, running a residence for priests studying and teaching in Rome. The Dominicans themselves conducted the excavations in the 1950s in collaboration with Italian archaeology students.