In the aftermath of the Battle of Actium (31 BC), Marcus Agrippa started an impressive building program: the Pantheon was a part of the complex created by him on his own property in the Campus Martius in 29–19 BC, which included three buildings aligned from south to north: the Baths of Agrippa, the Basilica of Neptune, and the Pantheon. It seems likely that the Pantheon and the Basilica of Neptune were Agrippa's sacra privata, not aedes publicae (public temples). The former would help explain how the building could have so easily lost its original name and purpose (Ziolkowski contends that it was originally the Temple of Mars in Campo) in such a relatively short period of time.
It had long been thought that the current building was built by Agrippa, with later alterations undertaken, and this was in part because of the Latin inscription on the front of the temple which reads:
or in full, "M[arcus] Agrippa L[ucii] f[ilius] co[n]s[ul] tertium fecit," meaning "Marcus Agrippa, son of Lucius, made [this building] when consul for the third time." However, archaeological excavations have shown that the Pantheon of Agrippa had been completely destroyed except for the façade. Lise Hetland argues that the present construction began in 114, under Trajan, four years after it was destroyed by fire for the second time (Oros. 7.12). She reexamined Herbert Bloch's 1959 paper, which is responsible for the commonly maintained Hadrianic date, and maintains that he should not have excluded all of the Trajanic-era bricks from his brick-stamp study. Her argument is particularly interesting in light of Heilmeyer's argument that, based on stylistic evidence, Apollodorus of Damascus, Trajan's architect, was the obvious architect.
Exterior of the Pantheon
The form of Agrippa's Pantheon is debated. As a result of excavations in the late 19th century, archaeologist Rodolfo Lanciani concluded that Agrippa's Pantheon was oriented so that it faced South, in contrast with the current layout that faces North, and that it had a shortened T-shaped plan with the entrance at the base of the "T". This description was widely accepted until the late 20th century. While more recent archaeological diggings have suggested that Agrippa's building might have had a circular form with a triangular porch, and it might have also faced North, much like the later rebuildings, Ziolkowski complains that their conclusions were based entirely on surmise; according to him, they did not find any new datable material, yet they attributed everything they found to the Agrippan phase, failing to account for the fact that Domitian, known for his enthusiasm for building and known to have restored the Pantheon after 80 AD, might well have been responsible for everything they found. Ziolkowski argues that Lanciani's initial assessment is still supported by all of the finds to date, including theirs; furthermore he expresses skepticism because the building they describe, "a single building composed of a huge pronaos and a circular cella of the same diameter, linked by a relatively narrow and very short passage (much thinner than the current intermediate block), has no known parallels in classical architecture and would go against everything we know of Roman design principles in general and of Augustan architecture in particular."
The only passages referring to the decoration of the Agrippan Pantheon written by an eyewitness are in Pliny the Elder's Natural History. From him we know that "the capitals, too, of the pillars, which were placed by M. Agrippa in the Pantheon, are made of Syracusan bronze", that "the Pantheon of Agrippa has been decorated by Diogenes of Athens, and the Caryatides, by him, which form the columns of that temple, are looked upon as masterpieces of excellence: the same, too, with the statues that are placed upon the roof," and that one of Cleopatra's pearls was cut in half so that each half "might serve as pendants for the ears of Venus, in the Pantheon at Rome".
The Augustan Pantheon was destroyed along with other buildings in a huge fire in the year 80 AD. Domitian rebuilt the Pantheon, which was burnt again in 110 AD.
The degree to which the decorative scheme should be credited to Hadrian's architects is uncertain. Finished by Hadrian but not claimed as one of his works, it used the text of the original inscription on the new façade (a common practice in Hadrian's rebuilding projects all over Rome; the only building on which Hadrian put his own name was the Temple to the Deified Trajan). How the building was actually used is not known. The Historia Augusta says that Hadrian dedicated the Pantheon (among other buildings) in the name of the original builder (Hadr. 19.10), but the current inscription could not be a copy of the original; it provides no information as to who Agrippa's foundation was dedicated to, and, in Ziolkowski's opinion, it was highly unlikely that in 25 BC Agrippa would have presented himself as "consul tertium." On coins, the same words, "M. Agrippa L.f cos. tertium", were the ones used to refer to him after his death; consul tertium serving as "a sort of posthumous cognomen ex virtute, a remembrance of the fact that, of all the men of his generation apart from Augustus himself, he was the only one to hold the consulship thrice." Whatever the cause of the alteration of the inscription might have been, the new inscription reflects the fact that there was a change in the building's purpose.
Cassius Dio, a Graeco-Roman senator, consul and author of a comprehensive History of Rome, writing approximately 75 years after the Pantheon's reconstruction, mistakenly attributed the domed building to Agrippa rather than Hadrian. Dio appears to be the only near-contemporaneous writer to mention the Pantheon. Even by the year 200, there was uncertainty about the origin of the building and its purpose:
Agrippa finished the construction of the building called the Pantheon. It has this name, perhaps because it received among the images which decorated it the statues of many gods, including Mars and Venus; but my own opinion of the name is that, because of its vaulted roof, it resembles the heavens.
—Cassius Dio History of Rome 53.27.2
In 202, the building was repaired by the joint emperors Septimius Severus and his son Caracalla (fully Marcus Aurelius Antoninus), for which there is another, smaller inscription on the architrave of the façade, under the aforementioned larger text. This now-barely legible inscription reads:
IMP · CAES · L · SEPTIMIVS · SEVERVS · PIVS · PERTINAX · ARABICVS · ADIABENICVS · PARTHICVS · MAXIMVS · PONTIF · MAX · TRIB · POTEST · X · IMP · XI · COS · III · P · P · PROCOS ET
IMP · CAES · M · AVRELIVS · ANTONINVS · PIVS · FELIX · AVG · TRIB · POTEST · V · COS ·PROCOS · PANTHEVM · VETVSTATE · CORRVPTVM · CVM · OMNI · CVLTV · RESTITVERVNT
In English, this means:
Emp[eror] Caes[ar] L[ucius] Septimius Severus Pius Pertinax, victorious in Arabia, victor of Adiabene, the greatest victor in Parthia, Pontif[ex] Max[imus], 10 times tribune, 11 times proclaimed emperor, three times consul, P[ater] P[atriae], proconsul, and
Emp[eror] Caes[ar] M[arcus] Aurelius Antoninus Pius Felix Aug[ustus], five times tribune, consul, proconsul, have carefully restored the Pantheon ruined by age.
An 1836 view of the Pantheon by Jakob Alt, showing twin bell towers, in place from early 17th to late 19th centuries.
In 609, the Byzantine emperor Phocas gave the building to Pope Boniface IV, who converted it into a Christian church and consecrated it to St. Mary and the Martyrs on 13 May 609: "Another Pope, Boniface, asked the same [Emperor Phocas, in Constantinople] to order that in the old temple called the Pantheon, after the pagan filth was removed, a church should be made, to the holy virgin Mary and all the martyrs, so that the commemoration of the saints would take place henceforth where not gods but demons were formerly worshipped." Twenty-eight cartloads of holy relics of martyrs were said to have been removed from the catacombs and placed in a porphyry basin beneath the high altar. On its consecration, Boniface placed an icon of the Mother of God as 'Panagia Hodegetria' (All Holy Directress) within the new sanctuary.
The building's consecration as a church saved it from the abandonment, destruction, and the worst of the spoliation that befell the majority of ancient Rome's buildings during the early medieval period. However, Paul the Deacon records the spoliation of the building by the Emperor Constans II, who visited Rome in July 663:
Remaining at Rome twelve days he pulled down everything that in ancient times had been made of metal for the ornament of the city, to such an extent that he even stripped off the roof of the church [of the blessed Mary], which at one time was called the Pantheon, and had been founded in honour of all the gods and was now by the consent of the former rulers the place of all the martyrs; and he took away from there the bronze tiles and sent them with all the other ornaments to Constantinople.
Much fine external marble has been removed over the centuries – for example, capitals from some of the pilasters are in the British Museum. Two columns were swallowed up in the medieval buildings that abutted the Pantheon on the east and were lost. In the early 17th century, Urban VIII Barberini tore away the bronze ceiling of the portico, and replaced the medieval campanile with the famous twin towers (often wrongly attributed to Bernini) called "the ass's ears", which were not removed until the late 19th century. The only other loss has been the external sculptures, which adorned the pediment above Agrippa's inscription. The marble interior has largely survived, although with extensive restoration.
Since the Renaissance the Pantheon has been the site of several important burials. Among those buried there are the painters Raphael and Annibale Carracci, the composer Arcangelo Corelli, and the architect Baldassare Peruzzi. In the 15th century, the Pantheon was adorned with paintings: the best-known is the Annunciation by Melozzo da Forlì. Filippo Brunelleschi, among other architects, looked to the Pantheon as inspiration for their works.
Pope Urban VIII (1623 to 1644) ordered the bronze ceiling of the Pantheon's portico melted down. Most of the bronze was used to make bombards for the fortification of Castel Sant'Angelo, with the remaining amount used by the Apostolic Camera for various other works. It is also said that the bronze was used by Bernini in creating his famous baldachin above the high altar of St. Peter's Basilica, but, according to at least one expert, the Pope's accounts state that about 90% of the bronze was used for the cannon, and that the bronze for the baldachin came from Venice. Concerning this, an anonymous contemporary Roman satirist quipped in a pasquinade (a publicly posted poem) that quod non fecerunt barbari fecerunt Barberini ("What the barbarians did not do the Barberinis [Urban VIII's family name] did").
In 1747, the broad frieze below the dome with its false windows was "restored," but bore little resemblance to the original. In the early decades of the 20th century, a piece of the original, as could be reconstructed from Renaissance drawings and paintings, was recreated in one of the panels.
Two kings of Italy are buried in the Pantheon: Vittorio Emanuele II and Umberto I, as well as Umberto's Queen, Margherita.
The Pantheon is in use as a Catholic church. Masses are celebrated there on Sundays and holy days of obligation. Weddings are also held there from time to time.