Roman emperors had all believed in the pagan deities, even when the early Christians began to spread their new faith. But In the IV century, the Empress of the Eastern Helena, converted to Christianity, found during her voyage in the Holy Land the relics of the Passion of God and placed them in the Roman Basilica of S. Croce in Gerulasemme.
This was the first step towards the edict of Constantine, son of the Empress, who in the meanwhile had been manoeuvring to become emperor of the West. Constantine's opportunity came in 312, when the Emperor of the West had died. At this point Constantine had to face the emperor of the West Maxentius. The night before the battle Constantine was visited by a miraculous apparition of a cross that read: "In Hoc Signo Vinces" ("in this sign you will win").
After his victory, the day after, he decided to convert to Christianity and soon after he promulgated an edict, the Edict of Milan, through which he guaranteed Christians freedom of worship. When Constantine became sole emperor of the empire, henceforth the "Great", he occupied the eastern headquarters in Byzantium and refunded it as Constantinople, proclaimed by him as the "New Rome".
The conversion of the emperor had also a political reason. In fact, Constantine had understood that he could restore stability and unity within the empire only making the followers of Christianity part of it. The recognition brought about many changes, including the edification of new religious monuments. Constantine promoted the erection of numerous churches, baptisteries, and the residence for the bishop of Rome (the Pope) at the Lateran.
He also helped to build several enormous funerary basilicas associated with martyrs' graves in the Christian cemeteries on the outskirts of the city: St Peter at the Vatican, St Sebastian on the Via Appia, and St Lawrence on the Via Tiburtina. The churches, besides celebrating the new religion, had a functional aspect. They worked as holy places for the gathering of the followers of Christianity.
Three of the Basilicas that have been erected under the reign of Constantine were among the majors of Rome.
St Peter in Vatican was founded in 324 to honour the first apostle, who was crucified in 67 AD under Nero. In front of the church, with a nave and five aisles, transept and apse, there was a four-sided portico with a bath for the followers to purify their feet.
In 1506 the Basilica, deeply ruined, was demolished by the architect Bramante and successively rebuilt under Pope Julius II by Michelangelo. To the other Apostle was dedicated the Basilica of San Paolo fuori le Mura, erected in the nearby of the gate Saint Paul and containing the tomb of the Apostle. It reflects the same plant of the first basilica, with a nave and four aisles.
A fire almost completely destroyed it in 1823. There survived only the mosaic of the transept, executed in 1220 by Venetian craftsmen, the chapel of the crucifix, and the splendid tabernacle by Arnolfo di Cambio, over the high altar. Also the inscription that reads: "Paolo Apostolo Mart" dates from the time of Constantine.
The last great basilica erected under Constantine is the Basilica of St John Lateran, in honour of Pope Silvestre. It was set up on the site of the barracks of the Imperial Guard and several wealthy houses. It was completely rebuilt in the 14th century, on the same basic plan, and remodelled in the middle of the 17th century.
The east facade dates from 1733-5. Despite the heavily Baroque overtones its huge five-aisled space (102 x 60 metres) gives some ideas of what one late Roman basilicas, especially the Basilica Julia on the Forum will have felt like. This basilica is the Cathedral of Rome, seat of the Pope.
Close to these churches raised also the octagonal Baptistery with, in the centre, the bath for the immersion of the adults to be christened encircled by eight red porphyry columns, and the Palazzo Lateran, that was pontifical seat until 1305.
The fourth Roman basilica is S. Mary Major, founded by Pope Liberius after the miracle of the snow, fallen on the Esquiline hill during the night of the 4th August of 352. During the 13th century, the apse of the church was enriched by precious mosaics by Jacopo Torriti.
After his death, the Roman people celebrated the emperor's victories over the Eastern Empire Maxentius with the erection of the magnificent Arch of Constantine. Before his total defeat, Maxentius had begun in 308 the construction of the great Basilica of the Roman Forum, dedicated to the political and legal affairs. At his death, Constantine terminated the erection of this rectangular church with one nave and two aisles.
Constantine's daughter, Constance, commissioned the erection of the Church of Saint Agnes, on the Via Nomentana, and, close to it, in 342, of the tomb for her and her husband. This circular Mausoleum then consecrated as church, represents one of the highest example of the architecture of the early Christianity, also thanks to the precious mosaics that decorate the apses and the vault.
Another architectural masterwork of this period is the Church of Saint Sabine, on the Aventine Hill, erected in 425 in the site where there was the house of this Roman matron.
One of the typical constructions of this period is the catacomb, where early Christians used to hide to escape the persecutions of the pagan emperors.
The catacombs worked both to perform the Christian rites and as tombs. The Christians used to gather in these deep underground galleries and clandestinely celebrated the holy rites and the burials. Inside there are still, well kept, frescos, urns, inscriptions, sarcophagus, and lanterns. The Catacombs of Callisto, along the Via Appia, are 20 metres long and structured on four floors. Since the 3rd century it became the place where popes were buried.
On the same street there are the Catacombs of St Sebastian, dedicated to the martyr killed by arrow shots. But the vastest Roman catacombs are those of Domitilla, under the Via Ardeatina, while in Via Salaria are placed those of Priscilla, where was discovered the oldest image of the Virgin Mary, painted in the 2nd century.
After Constantine, the Catholicity, menaced by new heretic religions, has undergone a period of crises. All the West Roman Empire was in crises, being increasingly menaced by invading barbaric peoples.
As a result of the increasing weakness of this region of the Empire, the eastern city of Constantinople (present-day Istanbul), elected by Constantine as new capital of the Roman Empire, replaced Rome in its political and religious leading rules. The western government was the transferred to Milan, Ravenna, and Pavia.
Rome suffered the invasions of the Goths, the Huns, and Vandals. In 476, the last emperor of the West Roman Empire, Romulus Augustulus, was deposed by Odoacer, king of a German tribe.
The empire was to be reunified in the 6th century by the eastern emperor Justinian, who fixed the capital in Bezant, the old Constantinople. In this period Rome deeply resented of the eastern influence. Its art and architecture are evident expressions of this influence.
Mosaics and sculptures remains the masterworks produced during the early Christianity and Byzantine periods. Among the most beautiful works are the sarcophaguses in the catacombs and the mosaics in the apses (S. Pudenziana, V century, SS. Cosma and Damiano, S. Teodoro al Palatino and S. Giorgio al Velabro, IV century, S. Agnes, VII century).
The churches that date from this period still kept the scheme typical of the civil Roman churches: a rectangular plant with two or four aisles, and an apse. Some churches had a circular plant, as S. Teodoro al Palatino, S. Costanza and S. Stefano Rotondo. The central plan was typical of the mausoleums and baptisteries because of their limited dimensions.