The construction of the old St Peter's Basilica (326 – c. 360)

The martyrdom of Peter in Nero's circus

In imperial times, the Vatican area, on the right bank of the Tiber and outside the centre of Rome, held gardens, villas and a great circus begun by Caligula and completed by Nero, the Circus Vaticanus.^[8] It was there, according to unanimous Christian tradition, that the apostle Peter was put to death during the Neronian persecution that followed the fire of Rome in 64, and crucified, tradition says, head downwards.^[9]

The apostle's body was buried very close by, in the necropolis running alongside the via Cornelia, on the northern flank of the circus.^[10] This topographical detail is decisive: it was not a late devotion that invented the burial place after the fact, but a continuous memory that fixed the tomb's location even before the Church was free to build.

The “trophy” of Gaius and the 2nd-century memory

The earliest written mention of the tomb goes back to around the year 200. A Roman priest named Gaius, in a polemic against a Montanist adversary, set against him the tombs of the founders of the Church of Rome. The passage is preserved by the historian Eusebius of Caesarea in his Ecclesiastical History (II, 25, §§ 6-7): “But I can show, I, the trophies of the apostles: if one goes to the Vatican or to the Ostian Way, one will find there the trophies of those who founded this Church.”^[11]

The Greek word tropaion (trophy) here denotes the funerary monument marking the tomb. The 20th-century excavations found precisely, beneath the altar, a small columned aedicule set against a red-plastered wall, datable to around 150-160: this is the “trophy of Gaius”, the fixed point around which the whole basilica was conceived.^[12] Tradition therefore did not designate a vague place; it handed down a precise address, verifiable in stone.

One tomb, not one site: the constraint that governs everything

To build there was a challenge. The hillside was occupied by a dense necropolis of still-used pagan mausoleums, and the ground sloped steeply. For the altar to fall exactly above the trophy, part of the hill had to be cut away, dozens of tombs buried and filled in, and a vast artificial platform created.^[13] No technical reason justified such labour: all that mattered was the will to set the church upon Peter's tomb, and upon it alone. This archaeological resolve, found intact by the modern excavations, is the best proof of the age and seriousness of the Petrine tradition.

Constantine, Sylvester I and the imperial decision

After his victory at the Milvian Bridge in 312 and the Edict of Milan (313), Constantine endowed Rome with several great Christian basilicas: St John Lateran, the pope's cathedral, then St Paul Outside the Walls and St Peter's. The building of St Peter's is attributed by tradition to the joint initiative of the emperor and Pope Sylvester I (314-335).^[14] The Liber Pontificalis, the official collection of the lives of the popes, reports that Constantine enriched the sanctuary with sumptuous gifts and had a great gold cross of 150 pounds laid upon the tomb, bearing a dedicatory inscription in the names of the emperor and his mother Helena.^[15]

The authorship of the building is a matter of scholarly debate. A recent study published in Vigiliae Christianae (2021), developing the arguments of Glen Bowersock and Alastair Logan, attributes the construction itself not to Constantine but to his sons — Constans (337-350) first and foremost, then Constantius II — on the evidence of the triumphal-arch inscription and the mosaics.^[16] Catholic tradition and most historians nevertheless uphold the Constantinian initiative: it was under Constantine that the decision was taken and the levelling begun.

A building site of some forty years

The exact dates remain debated, for lack of any surviving foundation document. The Encyclopaedia Britannica places the opening of the works between 326 and 333, with completion some thirty years later^[17]; the body of the English-language “Old St. Peter's Basilica” Wikipedia page (section “History”) favours an earlier start, between 318 and 322 — after M. Moffett, M. Fazio and L. Wodehouse, A World History of Architecture (2nd ed., 2008), p. 135 — with some forty years of work until a consecration around 360, while the infobox of that same page gives 326-333^[18]. This early dating is compatible with Gardner (Art Through the Ages, 2004 ed., p. 310), who places the start of the works “probably as early as 319”^[19]. The great specialist of Roman basilicas, Richard Krautheimer, proposed that the works may have been interrupted at Constantine's death in 337, then resumed around 357, at the time of Constantius II's visit to Rome, and completed in the 360s.^[20] Such a long duration is nothing surprising for a building of this scale, on such difficult ground.

The basilica: a civil model, not a temple

The chosen plan did not imitate Graeco-Roman temples, judged too bound up with pagan worship, but the civil basilica, the elongated hall of law courts and markets, like the Basilica Ulpia or the Aula Palatina.^[21] The choice is freighted with meaning: the Church did not settle into the mould of paganism; it adopted the form of the public assembly, the place where justice is rendered and the people gather. The Early Christian basilica was born there, and old St Peter's was its most prestigious model for the whole West.

The atrium, the “Paradise” and the bronze pine cone

One did not enter the church directly. The pilgrim first crossed a large arcaded atrium, the quadriporticus, called the “Paradise” (Paradisus), planted with greenery and adorned with an ablutions fountain.^[22] At its centre stood an enormous antique bronze pine cone, the pigna, flanked by bronze peacocks; there one purified one's hands before entering. This furnishing survived the demolition: the pigna still adorns the Belvedere courtyard in the Vatican, which owes to it its name of Cortile della Pigna.^[23]

This transitional space played a liturgical and symbolic role: it marked the passage from the profane world to the holy place. Five doors, including the famous doors of silver and then of bronze, then gave access to the church interior.^[24] The pilgrim, often come from very far, then entered one of the vastest roofed spaces of ancient Christendom.

A nave with five aisles

Inside unfolded five aisles: a broad central nave flanked by two side aisles on each side, separated by long rows of marble columns reused from ancient monuments (spolia), some twenty to a row.^[25] The building measured about 103 metres in length — some 122 metres with the atrium^[26] — and could hold three to four thousand faithful.^[27] The high walls of the nave, pierced with tall windows — eleven on each side according to the Encyclopaedia Britannica — flooded the space with abundant light and carried a cycle of frescoes: patriarchs, prophets, apostles and scenes from the Old and New Testaments.^[28]

The following table summarises the main features of the Constantinian building according to archaeological sources and ancient surveys.

The transept, the apse and St Peter's confession

At the far end, perpendicular to the nave, stretched a projecting transept — one of the first appearances of this cruciform arrangement, which would become typical of great churches. It ended in a semi-circular apse facing west, so that the facade looked towards Rome, in the east.^[29] At the junction, exactly above the tomb, stood the confession: Peter's memorial, which Constantine had encased in marble and red porphyry.^[30] A fenestella confessionis allowed the pilgrim to slip a cloth (brandeum) down to touch the relics, a practice attested by medieval sources.^[31]

The twisted columns and the memory of Solomon

Above the tomb rose an aedicule carried by famous twisted columns decorated with vine scrolls, which the Liber Pontificalis says were brought from Greece.^[32] The Middle Ages believed them to come from Solomon's Temple in Jerusalem, hence their name of “Solomonic columns”. The legend proved so tenacious that Bernini, in the 17th century, gave this twisted form to the colossal bronze columns of his baldacchino in the present basilica — thus handing down, into the new building itself, the visual memory of the old one.^[33]

Mosaics and inscriptions: the emperor at Christ's feet

The decoration was profoundly theological. The triumphal arch, at the entrance to the transept, bore a mosaic showing Constantine offering Christ the model of the church, beneath an inscription in Latin verse:

“Because, under your guidance, the world has risen triumphant to the stars, Constantine the victor has founded this dwelling for you.” — Inscription of the triumphal arch of the old basilica^[34]

The apse bore a mosaic depicting Christ enthroned between Peter and Paul, above a procession of twelve lambs figuring the apostles and two palm trees, symbols of martyrdom and victory over death.^[35] The whole decoration said the same thing: the Empire bows before Christ, and it is upon Peter's martyrdom that the Church of Rome rests.

The great pilgrimage sanctuary of the West

Throughout the Middle Ages, the old basilica was, with the apostles' tomb, one of the three or four greatest pilgrimage goals of Christendom, alongside Jerusalem and Compostela. People came to venerate Peter's tomb, to gain the indulgences of the jubilees — the first proclaimed by Boniface VIII in 1300 — and to leave offerings. The influx of the faithful shaped around it the whole Borgo quarter and the material organisation of the papacy.

The coronation of Charlemagne, Christmas 800

The most famous event to take place in the Constantinian basilica is the coronation of Charlemagne. On 25 December 800, before St Peter's confession, Pope Leo III set the imperial crown upon the head of the king of the Franks, whom the Roman people acclaimed “emperor of the Romans”.^[36] The place was not neutral: by having himself crowned on the apostle's tomb, Charlemagne bound his new power to the authority of Peter and his successor. The basilica was thus, for centuries, the favoured theatre of the great acts of Latin Christendom.

The Saracen sack of 846 and the Leonine City

In 846, a fleet of Saracen raiders sailed up to Rome and, unable to force the city walls, sacked the outer basilicas, including St Peter's, stripping the sanctuary of its treasures and profaning the apostle's tomb.^[37][38] To prevent such a sacrilege from happening again, Pope Leo IV had Vatican Hill ringed with a fortified enclosure, the Leonine City (civitas Leonina), built from 848 to 852, and restored the damaged parts of the building.^[39]

Medieval embellishments: Giotto's Navicella

Over the centuries, popes and donors enriched the building with chapels, altars, mosaics and works of art. The most famous was the Navicella, an immense mosaic commissioned from Giotto by Cardinal Jacopo Stefaneschi and executed at the beginning of the 14th century: it depicted St Peter walking on the water towards Christ, and filled the wall above the entrance arcade, facing the atrium.^[40][41] Giotto also made for one of the altars the great polyptych known as the Stefaneschi Triptych (around 1320), today kept in the Vatican Pinacoteca.^[42] These masterpieces attest the attachment of artists and popes to a sanctuary believed eternal.

The following chronology summarises the main stages of the building's life, from foundation to disappearance.

The excavations of Pius XII (1939-1950)

In 1939, the fitting out of a burial place for Pius XI in the Vatican Grottoes brought ancient structures to light. Pius XII then ordered systematic excavations beneath the high altar, carried out from 1940 to 1949.^[43] They revealed an intact Roman necropolis of the 2nd-3rd centuries, buried by the Constantinian levelling — confirming that Constantine had indeed built over a cemetery in use. At the lowest point, exactly beneath the altar, appeared the red wall and the small columned aedicule: the 2nd-century “trophy of Gaius”. On 23 December 1950, in his Christmas radio message, Pius XII announced to the world that the tomb of the Prince of the Apostles had been found, while specifying that none of the bones recovered could be identified with certainty as those of Peter^[44].

Margherita Guarducci and the “Petros eni” graffito

The inquiry took a turning point when the epigraphist Margherita Guarducci was entrusted with the study of a wall covered with Christian graffiti, the muro G, set against the trophy. On a fragment of plaster she deciphered, in Greek, Peter's name followed by three letters which she interpreted as “Petros eni”, Peter is here.^[45] She also established that bones, found in a niche of that wall and set aside in 1942 by the administrator Ludwig Kaas without archaeological procedure, came from that precise spot.

“Peter is here.” — Margherita Guarducci's interpretation of the muro G graffito, Vatican necropolis^[46]

The bones and Paul VI's announcement (1968)

The anthropological analysis by Professor Venerando Correnti concluded that the bones belonged to a single individual: a sturdy man of sixty to seventy — an age and build compatible with the apostle.^[47] At the end of these works, on 26 June 1968, Paul VI publicly announced that Peter's relics had been identified in a manner he judged convincing — thus lifting the reservation Pius XII had maintained in 1950 over the identification of the bones:

“New research, very patient and very precise, has been carried out, with the result that, supported by the judgement of competent and prudent persons, we believe we may hold it as positive that the relics of St Peter, too, have been identified in a manner we may hold to be convincing.” — Pope Paul VI, general audience of 26 June 1968^[48]

The Church has never imposed this identification as a dogma: the archaeologist Antonio Ferrua, who directed the excavations, said he was unconvinced that these bones were Peter's^[49], and the very fragmentary muro G graffito admits rival readings — Petros eni (“Peter is here”), but also, conversely, Petros endei (“Peter is not here”)^[50] — all the more so as those bones had been set aside in 1942 without archaeological procedure, as said above^[51]. But the convergence is remarkable: the 1st-century tradition, Gaius's testimony around 200, the 2nd-century trophy, the Constantinian levelling and the bones of a sturdy old man all point to the same spot — the one Constantine had chosen.

A basilica threatening ruin

After more than a thousand years, the building, poorly founded on made ground, showed alarming signs. When the popes returned from Avignon in the 14th century, the basilica was dilapidated; by the mid-15th century, the great south wall of the nave had already moved more than six feet — about 1.80 m — off the vertical.^[52] The humanist and architect Leon Battista Alberti, who examined the building under Nicholas V, gave a damning diagnosis:

“An extremely long and high wall was raised above a continuous series of openings, with no curve to give it solidity [...] ; [...] the continual force of the wind has already moved it more than six feet off the vertical.” — Leon Battista Alberti, on the nave of old St Peter's^[53]

The plan of Nicholas V

The first to envisage a rebuilding was Pope Nicholas V (1447-1455), who charged Bernardo Rossellino, probably advised by Alberti, with enlarging and strengthening the chevet of the basilica. The works, barely begun, stopped at the pope's death.^[54] The idea of a new building was launched, however: it would take half a century and a determined pope to bring it to fruition.

Julius II, Bramante and the first stone of 18 April 1506

Pope Julius II decided. At first concerned to install there his own mausoleum, entrusted to Michelangelo, he soon resolved to raze the ancient building in order to raise an entirely new one, to the plans of Donato Bramante.^[55] On 18 April 1506, Julius II solemnly laid the first stone of the present basilica, marking the beginning of the end for Constantine's work.^[56]

The decision was widely criticised: pulling down the most venerable sanctuary of the West offended the piety of many. Bramante, who demolished the ancient structures and their tombs without mercy, was nicknamed “Maestro Ruinante”, the master of ruin.^[57]

A demolition spread over a century

The old basilica did not vanish at a stroke. For decades, the two buildings coexisted: worship continued in the old part while the new site rose. Under Paul III, a partition wall was even raised across the nave to isolate the space still in use from the works zone.^[58] The demolition ended only at the beginning of the 17th century: the last original part of the Constantinian nave was pulled down in 1605, under Paul V, after a century of coexistence between old and new.^[59][60]

Of Constantine's work, almost nothing survives on site, but a few witnesses have crossed the centuries. The bronze pigna of the atrium still adorns the courtyard named after it in the Vatican. Two Renaissance papal tombs, those of Innocent VIII and Sixtus IV, sculpted by Antonio del Pollaiuolo's workshop between 1484 and 1496, were dismantled and reassembled in the new basilica — the only papal tombs of the old building to survive there.^[61] The confession and Peter's tomb, for their part, have not moved: the papal altar of the present basilica stands exactly above the 2nd-century trophy, in perfect continuity with the Constantinian choice.

The most visible witness is the Egyptian obelisk that once adorned Nero's circus, near the place of martyrdom. It had remained standing, south of the basilica, throughout the Middle Ages. In 1586, Pope Sixtus V had it transported by the architect Domenico Fontana to the centre of the new St Peter's Square, where it still stands.^[62][63] The old basilica is known in detail thanks to the surveys of canons Tiberio Alfarano (1571 plan) and Giacomo Grimaldi, who recorded its dimensions, altars and inscriptions before and during the demolition — precious sources without which Constantine's building would today be almost unreadable.^[64]

• The coronation of Charlemagne — the major event that took place in the Constantinian basilica, at Christmas 800.
• The construction of the present St Peter's Basilica — the Renaissance building that replaced the old basilica.
• Saint Peter — the apostle whose tomb governs the whole siting.