On June 2, 1672, the remains of Sts Innocent, Victor, Candidus and Laureatus were placed in the altar. Originally, the altarpiece was supposed to feature a painting portraying the life of St. Peter. Domenichino, the artist employed by the Congregation, was instructed to paint a picture of an episode of Peter's life. Domenichino came from the city of Bologna and was of the Bolognese School, a group of artists that also included Lanfranco and Ludovico Carracci. The topic originally chosen was taken from the book of Acts, in which Peter converts a Gentile to Christianity, one of the first such conversions. However, two groups had say over the decoration of the altar: The Congregation and the Chapter. The Congregation of the Fabricca of St. Peters, a group of cardinals and high-ranking churchmen, oversaw the construction of the new basilica. The Chapter of St. Peters were the group of priests who performed the sacraments and said mass in the basilica. The Chapter eventually proposed artwork illustrating the martyrdom of St Sebastian, believing that he was important enough to warrant a major chapel. The altar of St. Sebastian was painted by Domenichino between his commission in 1625 and 1631, when he left Rome for Naples. Domenichino's work took place under Pope Urban VIII, who reigned from 1623-44. Pope Urban VIII and his family, the Barberini, were known to have a personal connection to St. Sebastian, having patronized all three sites in Rome associated with him. In fact, the Barberini family chapel was built on the site where St. Sebastian's body was dragged from the Cloaca Maxima.

The painting represents St. Sebastian's martyrdom under the Roman Emperor Diocletian, usually dated in the year 288. Sebastian was a member of the Praetorian Guard, and fell victim to the emperor's persecution of Christians. Diocletian's was the largest persecution of Christians in the Roman period, and also the last before Constantine's adoption of Christianity. Sebastian fell victim to the persecution twice; the first time he was shot with arrows, but nursed back to health by St. Irene. After this, he criticized Diocletian to his face, condemning him for his treatment of Christians. For his efforts, he was beaten and thrown into the Roman sewer, the Cloaca Maxima. It should be noted that Domenichino was a painter and not a historian, as his painting combines elements of both events.

St. Sebastian was known as a plague saint, thought to have the power to ward off disease. According to legend, the erection of an altar in his name was enough to ward off a plague that ravaged Italy in the seventh century. As the Renaissance period was rife with plagues which often devastated whole communities, he was quite popular among Christians of that time. Specifically, in the 1620s, Rome lived in fear of a plague that was spreading throughout the Italian Peninsula. 10,000 people died in Florence alone, and veneration of St. Sebastian was sought in order to protect the Eternal City.

Sebastian was a popular subject among Renaissance painters, as many artists of that period, from Botticelli to Titian to Perugino, all painted him in some form. Sebastian's image in the painting, from the sign above his head to the full beard he sports, show a conscious effort by Domenichino to make the martyred saint look like the popular image of Jesus. As martyrs were thought to be emulating Jesus, Domenichino sought to mould Sebastian in his image.

In its day, the painting recieved much criticism, both from the public and from Domenichino himself. Many thought that the painting was too crowded, and Domenichino wrote that he hoped for a chance to re-do the painting, even to the point where he hoped the altar would fall into disrepair. Domenichino worked on the painting until 1631, when he left Rome in a hurry for Naples. There, the Neapolitans commissioned him to decorate the Naples Cathedral. Perhaps because of this, he recieved only 800 scudi (this being the currency of the papal states), and for the rest of his life he felt that he was underpaid. He claimed he ought to have recieved 1500 scudi, nearly twice what he was paid. For comparison, sculptors carving statues on the collonades were paid 80 scudi per statue. On the other end, Pietro de Cortuna recieved 5200 scudi for his work on the cartoons he created for the vestibule mosaics (he also felt that he was underpaid). Domenichino died in 1641, allegedly at the jealous hands of a group of painters known as the Cabal of Naples. This group was a triumvirate of painters who sought to eliminate competition from other artists. His dispute over payment did not end with his death, however, as his heirs continued to harass the Congregation for back payment.

In the cupola, above the vestibule and before the chapel, are mosaics featuring stories and legendary figures of the Old Testament, from Abel to Daniel. These were based on cartoons drawn by Pietro de Cortuna between 1652 and 1662. The mosaics depict a theme of martyrdom, where God in Heaven is worshipped by the martyrs. The cartoons on the lunettes were drawn by Raffaele Vanni between 1658 and 1663, and transferred onto mosaic by matteo piccioni. The mosaics on the spandrels represent Abel, Isaiah, Zechariah, and Ezekiel. Abel and Isaiah are the work of Guidubaldo Abbatini in 1656, while Zechariah and Ezekiel are the work of Bartolomeo Colombo, who made these between 1661 and 1662. Fabio Christofari constantly worked to maintain these mosaics, which were frequently detached as the linseed oil used to attach the stucco dripped to the floor due to the hot Roman summers.

More than a century after Domenichino, Pietro Paolo Christofari copied his St. Sebastian altarpiece onto a mosaic between 1730 and 1736. Like his father, Fabio, Pietro was a prominent Baroque painter and decorated many of the domes and altars that now adorn St. Peter's Basilica. His mosaic was modeled on a cartoon by the artist Giovanni Domenico Campiglia, made between 1725 and 1726. In 1730, Domenichino's original work, done on stucco, was relocated to the church of Santa Maria degli Angeli by Zabaglia. Zabaglia was a sampietrino, a skilled worker involved in the maintenance of St. Peters. He was also a famous inventor who specialized in transporting and preserving works of art.

On the right side of the chapel, there is a statue of Pius XI (1922-1939), the first head of state of Vatican City, after its creation in 1929 due to the Lateran Pact with Italy. This statue was made by the artist Francesco Nagni in 1949. On the left side, at the behest of the cardinals he created, there is a statue of Pope Pius XII, who reigned from 1939-1958 and oversaw the Second World War and the fall of Fascism. His statue was carved by Francesco Messina in 1964.

by Sam A Howard

Sources
The Altars and Altarpieces of New St. Peters, by Louise Rice
Saint Peter in the Vatican: The Mosaics and In the Sacred Space, ed. by Jaca Book, SpA
The Mosaics of Saint Peter's Decorating the New Basilica, F. R. DiFederico
The Oxford History of the Popes, by JND Kelly