The Baptistery of Constantine is located adjacent to the Basilica of St. John Lateran. This church is second only in importance to St. Peter's in Rome. The pope is also the bishop of Rome and his "home church" is here, not St. Peter's. Until the papal court was moved to Avignon in France, this was the main cathedral of Rome and the official residence of the popes was the adjoining Lateran Palace. Read Article: Baptistery of Constantine in Rome