The basilica of Santa Maria in Trastevere is one of the oldest churches in Rome, perhaps the first in which mass was openly celebrated. The basic floor plan and wall structure of the church date back to the 340s AD.
This is the queen of the churches in Trastevere. The inscription on the episcopal chair says that it is the first church dedicated to the Mother of God, although actually that privilege belongs to Basilica di Santa Maria Maggiore. In its foundation it is certainly one of the oldest churches in the city. A Christian house-church was founded here about 220 by Pope St. Callixtus I (217-222) on the site of the Taberna meritoria, an asylum for retired soldiers. The area was given over to Christian use by the Emperor Severus when he settled a dispute between the Christians and tavern-keepers, saying, “I prefer that it should belong to those who honor God, whatever be their form of worship.” In 340 Pope Julius I (337-352) rebuilt the titulus Callixti on a larger scale, and it became the titulus Iulii commemorating his patronage, one of the original twenty-five parishes in Rome; indeed it may be the first church in which Mass was celebrated openly. It underwent two restorations in the fifth and eighth centuries. In 1140-43 the church was re-erected on its old foundations under Innocent II The richly carved Ionic capitals reused along its nave were pillaged from the ruins of the Baths of Caracalla. When scholarship during the nineteenth century identified the faces in their carved decoration as Isis, Serapis and Harpocrates, a restoration under Pius IX in 1870 hammered off the offending faces.
The 13th-century mosaics in the apse are attributed to Pietro Cavallini.
The predecessor of the present church was probably built in the early fourth century although that church was the successor to one of the tituli, those Early Christian basilicas that were ascribed to a patron and perhaps literally inscribed with his name. Though nothing remains to establish with certainty where any of the public Christian edifices of Rome before the time of Constantine the Great were situated, the basilica on this site was known as Titulus Callisti, since a legend in the Liber Pontificalis ascribed the earliest church here to a foundation by Pope Callixtus I (died 222), whose remains, translated to the new structure, are preserved under the altar.
The present nave of this Romanesque church, rebuilt by Pope Innocent II (1138 –1148) and rededicated to the Virgin Mary, preserves its original basilica plan and stands on the earlier foundations. The 22 granite columns with Ionic and Corinthian capitals that separate the nave from the aisles came from the ruins of the Baths of Caracalla, as did the lintel of the entrance door.
Inside the church are a number of late 13th-century mosaics by Pietro Cavallini on the subject of the Life of the Virgin (1291) centering on a "Corontation of the Virgin" in the apse. Domenichino's octagonal ceiling painting, Assumption of the Virgin (1617) fits in the coffered ceiling setting he designed.
The façade of the church was restored by Carlo Fontana in 1702, who replaced the ancient porch with a sloping tiled roof— seen in Falda's view (upper right)— with the present classicizing one (below right). The octagonal fountain in the piazza in front of the church (Piazza di Santa Maria in Trastevere), which already appears in a map of 1472 , was also restored by Carlo Fontana. The image of Mary on the façade is believed to be the earliest iconographic depiction of the Virgin nursing Jesus.
The church keeps a relic of Saint Apollonia, her head, as well as a portion of the Holy Sponge. Among those buried in the church, are the relics of Pope Callixtus I and the body of Lorenzo Cardinal Campeggio.