In Rome, the eternal city lies one of the great cathedrals-The Basilica of St John Lateran (San Giovanni in Laterano)
From the Wikipedia entry on the ‘Avignon Papacy,’
Rome is called the Eternal City -because ancient Romans believed that no matter what happened to the world or how many empires came and collapsed, Rome would go on forever. This cathedral was dilapidated and in disrepair when the papal office transferred to Avignon but here is the eternal Basilica of St. John of Lateran as it stands now. The Avignon Papacy was the period from 1309 to 1377 during which seven successive popes resided in Avignon (then in the Kingdom of Arles, part of the Holy Roman Empire, now in today’s France) rather than in Rome.
The basilica of Saint John Lateran was built under pope Melchiade (311-314), it’s the most ancient church in the world. Due to the fact that the pope is also the bishop of Rome, Saint John in Lateran – being seat of the bishop’s residence – is also Rome’s Cathedral.