Located in the Lateran Palace in Rome, the Sancta Sanctorum is a chapel dedicated to St. Lawrence and used by the early popes for personal devotion. The chapel was meant to house a number of religious relics, the most important of which is the true portrait of Christ (the Acheropita) painted by St. Luke the Evangelist. The chapel is accessed through a staircase that originally led to Pilate's house and that Christ climbed to be judged. Called the Scala Sancta, the staircase was brought to Rome from Jerusalem by St. Helena, Constantine the Great's mother. Pilgrims today still climb the stairs on their knees while engaging in prayer. In 1278 a major earthquake destroyed the original chapel, and Nicholas III had it rebuilt and decorated. The new structure has a square plan capped by a crossribbed vault. Porphyry columns separate the apse from the nave and the floor features an elaborate Cosmatesque mosaic. The lower portions of the walls are covered in marble, while the upper parts feature frescoes by unknown artists. The scenes depicted illustrate the lives of the saints whose relics were kept in the chapel, including St. Lawrence. In the 16th century, during the reign of Sixtus V (r. 1585-1590), further decorations were added, mainly to the apse, as well as an inscription that reads "No holier place on earth exists," a reference to the holy relics originally kept in the chapel, some of which have since been moved to St. Peter's.
Historical dictionary of Renaissance art. Lilian H. Zirpolo. 2008.