The key to understanding Rome, and its layers of human endeavor, is to separate its 4 historic epochs into the 4 corresponding areas of the city.
Christianity came to Rome with St. Peter, who was crucified on Vatican hill in 64 AD.
It was declared the State Religion of the Roman Empire in 379 AD by Emperor Theodosius. And for 1,000 years the two centers of Roman Catholicism were the Vatican hill and St. John of Lateran hill (Rome's Cathedral).
We define Christian Rome as centered in the Vatican area, with St. Peter's Basilica and its fortress of Castel Sant Angelo, to which we add the adjacent Medieval quarter of Trastevere and the places of Catholic pilgrimage: basilicas, catacombs and leading Churches.
We date Christian Rome from the 5 C to the 14 C, roughly from the Fall of the Roman Empire to the eruption of the Renaissance: the Popes ruled the city and the hearts of all the faithful.
Christian Rome and its religious establishments throughout Europe preserved Western Civilization during the Dark Ages virtually and all representational art was religious. Where the Medieval candles burned the brightest was CHRISTIAN ROME.