The Pantheon survived so well because it’s been in continuous use for over 2,000 years. It went almost directly from pagan times to being a Christian church…a reminder that for its last century, ancient Rome was Christian.

For a fascinating glimpse of that early Christian history, we’re heading outside the city for a look at the catacombs. Rome’s ancient wall stretched 12 miles. It fortified the city for centuries. From gates like this, grand roads fanned out to connect the city with its empire.

The Appian Way — Rome’s gateway to the East — is fun to explore on a rented bike. It was the grandest and fastest road yet…the wonder of its day. Very straight — as Roman engineers were fond of designing — it stretched 300 miles past Naples and on to Brindisi, from where Roman ships sailed to Greece and Egypt. These are the original stones.

Tombs of ancient big shots lined the Appian Way like billboards. While pagans didn’t enjoy the promise of salvation, those who could afford it purchased a kind of immortality by building themselves big and glitzy memorials. These line the main roads out of town.

But of course, early Christians didn’t have that kind of money. So, they buried their dead in mass underground necropoli — or catacombs — dug beneath the property of the few fellow Christians who did own land.

These catacombs are scattered all around the city, just outside the walls. And several are open to the public. Descending, you enter an ancient and sacred world. The tomb-lined tunnels of the catacombs stretch for miles and they’re many layers deep. The countless niches were considered only resting places, where the dead awaited their day of resurrection. Tombs were sealed by clay or marble slabs carved with humble Christian symbolism. Many of the first Christians buried here were recognized as martyrs and saints. Others then carved out niches nearby to be buried close to these early Christian heroes.

By the Middle Ages, the catacombs were abandoned and forgotten. Centuries later they were rediscovered. Romantic Age tourists on the “Grand Tour” visited by candlelight and legends grew about Christians hiding out to escape persecution. But the catacombs were not hideouts. They were simply budget, underground cemeteries.

Christianity would ultimately replace the Roman Empire as the driving force of European culture. In the year 313, the religion actually became legal. That’s when the general Constantine, following a vision that he would triumph under the sign of the cross, beat his rival for the throne of Rome. Taking power as emperor, Constantine then legalized Christianity. This obscure outlawed Jewish sect eventually became the religion of the empire.

In the year 300, you could be killed for being a Christian. In the year 400, you could be killed for not being a Christian. Church attendance boomed, and Emperor Constantine built the first great Christian church right here — San Giovanni in Laterano…St. John’s.

It opened as a kind of “first Vatican.” St. John’s — which has been rebuilt over the ages — was the original home of the bishop of Rome, or “Pope.” High atop the canopy over the altar, a box contains the supposed skulls of the first bishop, Saint Peter, and the Apostle Paul. The church is filled with symbols of Christianity’s triumph over pagan Rome…like these gilded bronze columns said to be from pagan Rome’s holiest temple. And what better entrance for this first grand church than these massive bronze doors which once hung in ancient Rome’s Senate House.

The adjacent Holy Stairs are a major stop on Rome’s pilgrimage trail. Many credit Emperor Constantine’s devout mother, St. Helena, for her son’s conversion. She brought home wagonloads of relics including these stairs — believed to be from the palace of Pontius Pilate. For 1,700 years, pilgrims — believing Jesus climbed these stairs the day he was condemned — have scaled the Scala Santa on their knees.

By the year 500, the over-expanded, corrupt, and exhausted Roman Empire had fallen. But the grandeur of the Roman Empire lived on in the Roman Church. Over time, Trajan’s column was capped with a Christian saint, the Pantheon became a church, Emperor Hadrian’s mausoleum became the Pope’s fortress, and the tomb of the Apostle Peter, a man the Romans executed, was crowned by the grandest building in the city — St. Peter’s Basilica.