The Pola Casket, ca. 400–430, ivory relief showing Constantine’s shrine of Peter at the Vatican with its six-column baldachino
(Photo: Istituto centrale per il catalogo e la documentazione, E. 51229)
Plan showing St. Peter’s Basilica under Pope Sixtus III (432–440) (adapted from S. de Blaauw, Cultus et decor, 1994, Fig. 19)
Longitudinal section of the Pelagian/Gregorian shrine to Peter at the Vatican around 600 (adapted from S. de Blaauw, Cultus et decor, 1994, Fig. 24)
It has been said that the long domination of the visual arts by Christianity in western Europe between the beginning of the fifth century and the beginning of the sixteenth century actually started, appropriately, with Constantine’s construction of a martyrium over the tomb of St. Peter (begun 317-322). This is an exaggeration but the Old Basilica may have been one factor for such domination.
The church was built in the style of a large Roman martyria basilica according to a plan that a good many churches in subsequent centuries throughout Europe and elsewhere would imitate, especially those wishing to stress a close association with Rome. (See Louise Gardner, Art Through the Ages, Ninth Edition, (Fort Worth, Harcourt Brace Jovanovich College Publishers 1991) p. 258)
For 1200 years the Constantinian Basilica developed and its changes were mirrored in Church architecture elsewhere.
The original basilica of St. Peter’s was intended, like all the other martyria, as a shrine or memorial to the martyr buried there —in this case, St. Peter.
A memorial had marked Peter’s grave since the middle of the second century.
A new memorial was constructed around the original one (a confession). The carving on the rear of the famous ivory casket from Samagher near Pola in Istria (discovered 1906) provides the best evidence for the appearance of this shrine
In order to accommodate the large crowds of pilgrims wishing to get close to the tomb, a nave or hall perpendicular to the central nave and side aisles, ran across the width of the basilica just in front of the apse, extending slightly beyond the width of the building on both sides.
The memorial was in the apse side of this space, right in front of the apse.
The continuous transept feature became a common element in many of the larger churches throughout Europe and the Near East.
That the resulting ground plan looked like a cross (T plan) was co-incidental and was not at all a factor in its use.
Gregory Nazianzen, in 380, was the first to observe the semblance to a cross in such plans which is about the same period of time when the unambiguous cross was coming into its own as a Christian symbol. The accidental cross shape resulting in the use of transepts in church plans would eventually be seized upon and its symbolic power exploited in subsequent periods of church architecture.
Like the other martyria, the nave of the original St. Peter’s was once paved with graves and functioned as a funerary banquet hall.
It seems that the original St. Peter’s (completed ca. 360) had seven altars and could hold over 14,000 people. Twenty-two marble columns (spoils from earlier pagan temples) separated the side aisles from the central nave.
The church proper was 350 feet long, 215 feet wide, and covered by a roof that is estimated to have been 100-125 feet off the ground; higher than some gothic cathedrals.
A large baldichino ciborium (canopy) supported by four twisted columns, covered the memorial. Two additional columns on either side of the baldichino joined the canopy to the sides of the front of the apse. Curtains were suspended between these columns and concealed the interior of the apse.
Most basilica churches had an enclosed courtyard—an atrium— immediately in front of the main doors. St. Peter’s was no different. Its atrium was completed in 390 and was known as “the Garden of Paradise.” In the centre stood a fountain used for washing before entering the church. Those people who had not yet been baptised received their instruction in the atrium, under the covered archways surrounding the open yard.
St. Peter’s also had a fairly large propylaeum or entrance gate, completed in the sixth century, on the front side of the atrium, facing the street.
Some claim that the original model for St. Peter’s was drawn from the Biblical description of Solomon’s Temple in Jerusalem
At the start the Vatican complex had functioned as a monumental Christian cemetery. But during the second half of the fourth century, the popes moved their Christmas celebrations from the Lateran to the Vatican.
By the early fifth century, moreover, the popes were also coming from the Lateran to St. Peter’s to celebrate the related feast of Epiphany, newly introduced in Rome from the East. See de Blaauw, Cultus et decor, 55 and 434 (with sources).
St. Peter’s thus appears to have been the first sanctuary in Rome to share papal masses with the Lateran.
Of the later history, Judson J. Emerick in Altars Personified: The Cult of the Saints and the Chapel System in Pope Paschal I’s S. Prassede (817–819) writes:
"In the nearly four centuries that had passed between the reigns of Popes Sixtus III and Paschal I, the Roman people’s perception of the great church at the Vatican had again changed radically. If in the 430s, Sixtus III had used the temporal liturgy, that is, the papal stational liturgy, to transform Constantine’s old cemetery complex into a full-fledged church in a papal system of churches, then a few generations later, in the early sixth century, Pope Symmachus exploited the rapidly growing cult of the saints, that is, the sanctoral liturgy, in a new attempt to remake St. Peter’s – to make it over into a cathedral. ...
St. Peter’s under Symmachus had already become a church with a main memorial to Peter accompanied by a number of similar, supplementary memorials to other saints. ...
[D]uring the course of the seventh and eighth century, but mostly in the eighth, the popes used a new liturgical tool, the reliquary altar, to transform St. Peter’s into a church focused on the worship of the saints, a church that had a main shrine to Peter and many secondary ones to other important saints in side chapels, and a church in which the people’s access to the sacred in all the shrines was under clerical, indeed papal, mediation."