The monument stood at the center of the transept within a great basilica, measuring 110 meters by 55, begun around the year 322 A.D.—a decade or so after the famous edict that granted religious freedom to the Christians. Constantine’s veneration for Peter’s tomb was such that he did not hesitate to embark on what was, by any standards, an architectural folly: building an immense construction largely on clay, at the slopes of a hill that would have to be entirely excavated. So strong was the Emperor’s religious sentiment that he risked profaning a necropolis that was still in use.
It is a touching thought for us today to know that the level of the new construction was determined by that humble slab of marble marking Peter’s tomb. All the mausoleums that stood higher than the memorial were either opened or destroyed; those which were lower were covered with earth in the same manner that Pompeii and Herculaneum were buried beneath the lava of Vesuvius.
And that was the way the pre-Constantinian necropolis remained until the 1940s, when it was excavated by order of Pius XII. It constituted one of the great archaeological discoveries of our time. Peter’s tomb was the ideal cornerstone for Constantine’s basilica and—twelve centuries later—for the even more grandiose basilica which succeeded it. The connection between then and now is
something that is almost palpable. There is a continuity of tradition, an unchanging veneration for that tomb—which has now been identified with absolute certainty.
In the first centuries after Christ, Constantine’s basilica was essentially a combination temple-and-mausoleum dedicated to Peter. Whereas the transept was given over to the veneration of the tomb, the nave and aisles were reserved for liturgical activities. If the celebration of Mass was called for, a portable altar was put there.
At the end of the sixth century, it was St. Gregory the Great who called for the construction of a permanent altar. He had the presbytery raised, thereby creating a semicircular crypt which permitted the faithful to approach the tomb virtually on their knees. In his turn, Calixtus II created a second altar above that of Gregory in 1120. And above even that, in the new basilica, the papal altar (known as the “confessional altar”) was placed vertical to the original tomb. It was consecrated by the Aldobrandini pope, Clement VIII, in 1594.
The same pope ordered the alteration and embellishment of the crypt. Still today we can walk along the aisle that leads to the chapel (named after Clement) in the Vatican grotto. So it is clear that the veneration of Peter’s tomb is something which has united all his successors— right up to Pius XII’s ordering fresh excavations a half century ago and John Paul II’s decision to render Peter’s
monument more visible to pilgrims by making a new entrance beneath the Confessional Altar.