After St. Peter died upside down on a cross in the Circus of Caligula and Nero, the surviving Christians obtained his body and buried him quickly nearby, on the steeply sloping Vatican Hill to the north of the Circus. That hill had become a makeshift graveyard four months earlier after the fire of Rome had killed so many residents of the metropolis that their loved ones began to use any open spot they could find on the roadsides radiating outside the city.

The Christians buried Peter in a simple “poor man’s grave,” which consisted of a shallow hole in the ground, where they placed the apostle’s body, and covered it with a series of six terracotta tiles in the form of a gable. Since Peter’s pauper’s plot was on the side of a heavily inclined hill, it was in serious danger of being destroyed by erosion when the rains came. So the early Christians on two separate occasions built small, primitive brick walls around the tomb as protection. This was the way the prince of the apostles’ grave stood for almost a hundred years.

In the middle of the second century, the Romans wanted to improve the whole Vatican burial site by digging up the scattered graves, putting the remains in ossuaries, and constructing an organized necropolis, or “city of the dead.” They gave those with loved ones buried on the hillside the chance first to build worthy mausoleums or funerary monuments over their kins’ tombs as part of the new complex. The Christians likewise needed to build something over Peter’s grave or lose it. They opted to build a tropaion, literally a “victory monument” or “trophy,” which was generally used by pagans for those they thought had lived a “victorious” or “happy” life. It consisted of two marble columns, about four feet high, stretching from a marble base to a marble ledge. The back wall was made of brick and covered with red plaster running eight feet high The part of the wall between the columns in the center of the monument had concave niches in which the pagans, in typical tropaia, were accustomed to put urns with the cremated remains of their loved ones. The Christians also would have built walls to make a room open to the sky in front of the victory monument.  

Why did the Christians choose a pagan cremation monument for Peter’s grave? In order to throw off the scent of bloodthirsty anti-Christian emperors and soldiers, since Christianity was a crime punishable by death. The disciples wanted to do something quasi-permanent over Peter’s grave without doing anything conspicuously Christian. The fact that they built a tropaion over Peter’s grave was attested in literary sources as early as 199 in a letter by a Roman priest named Gaius.

About 100 years after the tropaion was built, the red wall behind the monument developed a crack in it. To keep the edicule standing during a time of ferocious anti-Christian persecutions, the disciples built an inelegant but functional buttressing wall off the side of the northern column.

Thus St. Peter’s humble tomb remained for another 60 years, until the emperor Constantine legalized Christianity in response to his triumphing in the Battle of the Milvian Bridge in a victory he ascribed to the intervention of Jesus Christ.

To thank the Christian God whom he deemed had just given him the western half of the Roman empire, Constantine first legalized Christianity in 313 so that who worshipped this very generous God could no longer be killed for doing so. But as soon as Christians could practice their faith with impunity, they were coming in huge numbers to pray at the tombs of the saints. In the Vatican, Peter’s tiny victory monument, held together by an ugly buttressing wall, could not withstand the crowds.

So Constantine decided to build an enormous basilica over the place where St. Peter was buried for the Christians to pray. He had two problems, however. The first was that it was a crime punishable by death to desecrate a grave — and Constantine wasn’t just intending to destroy a grave but to decimate a graveyard. As emperor, of course, he could simply change the law, but he knew he would be doing so against the sacred sensibilities of the Roman pagans. The second obstacle was that the necropolis had been build on the side of a steeply sloping hill. To level it to make a basilica while leaving the grave of Peter essentially intact meant the largest building project of the 4th century, moving several million cubic feet of dirt to create a plane on which to build his sacred edifice. Constantine decided to brave both obstacles because he deemed it important enough to have this basilica symbolize the Church as a whole, which Jesus promised he would build on Peter (Mt 16:18). His basilica lasted 1200 years. It remains essentially the crypt of the present St. Peter’s, built between 1506-1626.  

Constantine enclosed Peter’s collapsing victory monument in a marble and porphyry covered box at the intersection of the transept and nave. Eventually Pope St. Gregory the Great built up the area behind the box so that he could celebrate Mass directly on top of Peter’s tomb. Five hundred years later, Pope Callistus II erected another altar on top of Gregory’s and almost five hundred years after that, Pope Clement VIII, in 1592, built the altar on which Pope Benedict continues to celebrate Mass to this day. St. Peter’s basilica, therefore, still represents the Church as a whole, built literally on top of Peter.

When the tropaion of Peter was found underneath the high altar during archaeological escavations in 1941, there was great rejoicing, because it matched what Gaius had written at the end of the second century. Even more exciting was the fact that they found bones in what was clearly Peter’s tomb underneath the victory monument.

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