The Lateran Baptistery (San Giovanni in Fonte al Laterano) was built in the early 4th century. Its octagonal design was copied in numerous baptisteries throughout the world; in the Bible the number eight represents a new beginning.
The eight porphyry columns in the interior were erected by Pope Sixtus III (r. 432-40). They support an architrave, which, in turn, supports eight smaller marble columns. In the centre is the font, an ancient green basalt bath-tub. It stands in a circular depression, a reminder of the original baptismal plunge-pool.
The beautiful gilded and painted wooden ceiling dates back to the reign of Pope Urban VIII (r. 1623-44). The drum of the dome has eight frescoes by Andrea Sacchi (1599-1661). In the centre of the vault is the Dove of the Holy Spirit in glory. It is surrounded by a quotation from the Book of Genesis: SPIRITVS DEI FEREBATVR SVPER AQVAS(The spirit of God moved upon the face of the waters, Gen. 1:2, KJV). The small chapel of St John the Baptist dates back to the 5th century. The interior was entirely remodelled by Giovanni Battista Ceccarelli in 1780. The bronze statue of St John the Baptist is by Luigi Valadier, the father of the more famous Giuseppe Valadier. Above the entrance are two inscriptions: ERVNT ASPERA IN VIAS PLANAS(The rough places will become level ways, Luke 3:5) and HILARVS EPISCOPVS PLEBEI DEI (Bishop Hilary to the people of God). The doors to the chapel, which date back to the 5th century, bear the inscription: IN HONOREM BEATI IOHANNIS BAPTISTAE HILARVS EPISCOPVS CHRIST FAMVLVS OFFERT (Bishop Hilary, the servant of Christ, offers in honour of John the Baptist). The doors were once famous for the musical sound they made when moved. Unfortunately, this stopped after the Mafia detonated a bomb outside the baptistery on July 27th, 1993. The Cappella di San Venanzio(Chapel of St Venantius) was added by Pope John IV (r. 640-42) .