Basilica of St. John Lateran, the highest-ranking and oldest among the great papal basilicas of Rome as well as the oldest public church in Rome, having been consecrated in 324 CE by Pope Sylvester I. Though it is outside Vatican City proper, it nonetheless belongs to the Holy See.
St. John Lateran rests on what used to be the palace of the Laterani family, whose members served as administrators to several Roman emperors. Around 311 it came into the hands of Emperor Constantine I; he later gave it to the bishop of Rome. In 313 it hosted a council of bishops who met and declared the Donatist sect to be heretical. From its consecration in 324, the basilica, together with the adjoining Lateran Palace, was the centre of Christian life in the city, the residence of popes, and the cathedral of Rome.
The original church was probably not very large, and it was dedicated to Christ the Savior (Christo Salvatori). It has been rededicated twice—once in the 10th century to St. John the Baptist, and again in the 12th century to St. John the Evangelist. In popular usage, these subsequent dedications have overtaken the original, though the church remains dedicated to Christ, like all patriarchal cathedrals. In 1309, when the seat of the papacy was moved to Avignon in France, the basilica began to decline. It was ravaged by fires in 1309 and 1361, and, although the structure was rebuilt, the original splendour of the building had been destroyed. Because of this, after the papacy returned to Rome, the Vatican Palace was constructed in 1589 as the new papal seat.
In the late 16th century, Pope Sixtus V ordered the basilica to be torn down and a replacement built, a project overseen by Domenico Fontana—another in a long continuing line of refurbishments and rebuildings of this most important of cathedrals. Further renovations took place under Pope Innocent X in the 17th century and under Pope Clement XII in the 18th century; the facade was completed in 1735. Despite being bested in architectural terms by St. Peter’s Basilica, which holds most papal ceremonies thanks to its size and location within the Vatican’s walls, St. John Lateran remains the cathedral church of Rome and the official ecclesiastical seat of the pope as bishop of Rome. All five Lateran Councils met there, and it was the location of the signing of the Lateran Treaty. Indeed, it is the mother church of Roman Catholicism worldwide.