Constantia was probably born in 312 -- the year Constantine marched on Rome and triumphed over Maxentius at the Battle of the Milvian Bridge. She was therefore older than all three of her full brothers, who were born in 316 (Constantine II), 317 (Constantius), and probably 323 (Constans). Constantia makes her first appearance in the historical record while still a teenager. The biography of Pope Sylvester I (314-335) in the Liber Pontificalis records that sometime in the late 320's, at the request of his daughter [Constantia], [Constantine] built a basilica to the holy martyr Agnes and a baptistery in the same place, where his sister Constantia, together with the daughter of the Augustus, was baptised by Bishop Sylvester. The cemetery basilica of St. Agnes was erected on an imperial estate owned by Constantine some three kilometers northeast of Rome on the Via Nomentana. The apsed basilica itself still stands, albeit in a ruinous state, while the remains of the triconch baptistery, which was attached to the southwest wall of the basilica, have recently been identified beneath the later imperial mausoleum now known to us as Santa Costanza.

We can therefore imagine the elder Constantia, the widow of the eastern Emperor Licinius whom Constantine had vanquished in battle in 324 and then executed two years later, following her teenaged niece and namesake into the baptismal font to receive the sacrament. Hanging above the font was a magnificent gold lantern with twelve wicks, a gift from the Emperor Constantine that is also recorded in the Liber Pontificalis. The ceremony may have been held outside the city in the small baptistery on the Via Nomentana property to avoid offending the sensibilities of the still heavily pagan upper classes in Rome. In any case, the ceremony must have been freighted with very different significance for the teenaged girl, who appears ardent and pious in the account given by the Liber Pontificalis, and her middle-aged aunt, who until a few years earlier had been the wife of an eastern Emperor known for his hostility to Christianity.

The next reference to Constantia in the historical sources comes in 335, when she married Hannibalianus, a nephew of Constantine the Great's and the son of his half-brother Dalmatius the Censor. In the final years of Constantine's reign, Hannibalianus was appointed governor of Pontus and Roman Armenia and assigned the title Rex Regum, suggesting that Constantine may have planned to place him and his daughter on the throne of the border state of Armenia once his planned campaign against the Persians was successfully concluded. Within less than three months after Constantine died, however, in August 337, Hannibalianus was one of ten members of the families of Constantine's two half-brothers and closely associated members of the imperial court who were seized and executed in a military uprising.

It is unclear whether this uprising was actually initiated on the orders of Constantine's second surviving son, Constantius II, or whether he was merely a passive beneficiary of the murder of so many of his close relatives, which removed them as rival candidates for the throne. Did Constantius plot the doom of his own sister's husband? If so, did Constantia know of her brother's murderous designs in advance? Or was she instead surprised by the sudden arrest and execution of her husband? If the latter, did she credit the story put out to justify the executions, which claimed that Hannibalianus and the others had poisoned Constantine? We know two things only: That after her husband was in his grave, Constantia did not remain in the east, but returned to Rome, and there lived as a widow for the next thirteen years. These bare facts may nevertheless suggest that she felt a real and genuine grief at her young husband's death, and did not accept the stories put forth to the public to justify his execution.

When she reappears on the historical stage thirteen years later, we again find Constantia playing an ambiguous part. In early 350, during a time of turmoil triggered by the revolt of the usurper Magnentius in Gaul, the respected general Vetranio, commander of the Danube frontier, also declared himself Emperor. Gibbon tells us that Vetranio's ambition derived a specious pretence from the approbation of the princess Constantia . . . [who] placed the diadem with her own hands on the head of the Illyrian general. But all may not be as it first appears. Vetranio soon agreed to resign the purple in favor of the eastern Emperor Constantius II, who then pardoned him and pensioned the old general off to a comfortable retirement. Some historians have concluded that he was either temporarily coerced into declaring himself Emperor by his troops, or that, knowing the restless temper of his men, he did so lest some other genuine rebel be selected. So, again we are left to wonder. Did Constantia co-operate with her brother Constantius and Vetranio in a carefully stage-managed charade? Or did she aspire to become Empress of the West after the death of Constans I, hoping to use Vetranio as her instrument?

What we know is that Constantia does not seem to have been out of favor with her brother Constantius following the Vetranio episode. This demonstrated by the events of the following year, when he selected her to marry the Caesar Gallus, the eldest son of Julius Constantius (and thus the elder brother of Julian the Apostate). Constantius had plucked Gallus out of a luxurious prison near Caesarea to assume responsibility for the Empire's eastern frontier, with his residence at Antioch. We must suspect that Constantia's marriage to him was both a diplomatic move and a means of keeping a close eye on him. Once in his new position, the insecure Gallus filled the cities with spies and informants and launched a reign of terror in which Constantia is said to have been an eager and greedy accomplice. Ammianus Marcellinus relates a story about how she agreed to procure the execution of an Alexandrian nobleman in return for a pearl necklace. Again, however, the story is suspect. Ammianus says that the nobleman in question, one Clematius of Alexandria, got into trouble because he refused his mother-in-law's sexual advances, and the scorned woman, angry and humiliated, determined to make an end of him. But this suspiciously echoes the story of Joseph and Potiphar's wife, a tale that seems to have an enduring hold over the popular imagination in the middle ages (a similar story is told in connection with the death of the Latin Emperor Baldwin I in Bulgarian captivity, for example). The story's recurrence in other historical contexts is likely explained by its incorporation of a trifecta of traditional male fears about women, whom it depicts as sexually voracious, furiously vengeful, and Odyssean in their wiliness and capacity for scheming.

Eventually, Constantius demanded that Gallus appear at his capital of Adrianople to answer charges brought against him by his disgruntled subjects. Constantia preceded him on the journey, apparently hoping to intercede with her brother and soothe his anger; but she died on the way at a small village in Bithynia, sometime in the fall of the year 354. Without Constantia to placate Constantius, Gallus's fate was sealed. When he arrived in Adrianople, he was stripped of his position, imprisoned in a gloomy fortress at Pula at the head of the Adriatic, and afterwards beheaded.

Constantia's own body was ultimately returned to Rome and to the estate on the Via Nomentana for burial. But her story was not ended yet. Six years later, her younger sister Helena, the wife of the Caesar Julian, died in childbirth. There is every reason to believe that the imposing circular mausoleum that we now know as the Church of Santa Costanza was built by the Caesar (and soon-to-be-Emperor) Julian as a tomb for his dead wife. In order to make room for the new imperial mausoleum, the small triconch baptistery was leveled, and Helena's tomb rose in its place. Once it was finished, the two imperial sisters were both re-interred there.

The Nave (the central dome-covered space with its surrounding annular aisle) of Santa Costanza

The columns and annular aisle of Santa Costanza, showing the surviving fourth century mosaic decoration with its mixed pagan and Christian themes in the vault (photograph © George Mastallone, members.aol.com/zorzim)

Later, when Constantia was canonized by the Church, the mausoleum was re-dedicated in her honor and transformed into a church (1254) -- although that did not save her from the indignity of having her porphyry sarcophagus recycled in 1606 to house the relics of St. Simon and St. Jude in their chapel in the Vatican, where it may still be seen in the south transept of St. Peter's. A pair of monumental marble candelabra from the old mausoleum, standing almost six feet high and carved with vegetation, garlands, and sphinxes, the traditional guardians of a pagan tomb, also survive in the collections of the Vatican Museum. Finally, the Church of Santa Costanza itself still stands to this day, one of the finest paleochristian monuments of late antiquity. The Church is particularly famous for the splendor of its surviving mosaic decoration, which appears to incorporate both Christian and pagan elements -- thereby possibly memorializing the tension felt by the Emperor Julian, a convinced pagan for more than a decade, as he prepared a monument to receive and honor the remains of his devoutly Christian wife and sister-in-law.

Constantia’s massive porphyry sarcophagus, preserved today in the Vatican Museum

References:

Jacob Burckhardt, The Life and Times of Constantine the Great (1949), at 285-91

Edward Gibbon, The Decline & Fall of the Roman Empire (J.B. Bury ed.), Volume II, at 247-67

Ammianus Marcellinus, The Later Roman Empire: 354-378 A.D.