With the adoption of Christianity as the official religion, art was able, so to speak, to come above ground in the old pagan city of Rome, and painting, instead of being restricted to the decoration of the walls of the Catacombs or of small chambers and chapels, came into use on a large scale in the new churches that were at once set up. At the same time patronage moved from the hands of the poorer classes to the richer, and artists of outstanding quality came to be employed as well as those of obscurer character, who would work for small fees. To wall painting was added the more luxurious art of mosaic; numerous sculptures were done, and minor objects, often in expensive materials, were in addition produced in the service of the Church, so that art production became at the same time both more extensive and more luxurious.

A great deal of the work that was done at this time has of course perished, more especially that in fragile materials, such as textiles or paintings on panels, but a few mosaics of the fourth century and a good many more of the fifth survive in Rome, and there is quite a lot of sculpture, both on a large scale in stone and on a small in ivory. Something has already been said about the ivories, more especially the Consular diptychs, which necessarily form a part of the general picture, though it is not always easy to be sure of where they were made, as they are in a diversity of styles. Here we are concerned not so much with these things as with works which are essentially Christian and also undoubtedly Roman, such as the mosaics and wall painting, which are necessarily immovable, or stone sculptures on a large scale in a material which was carved on the spot and quarried in the neighborhood.

The earliest of the mosaics are those in the church of Sta Constanza, which was built as an octagonal martyrium or tomb sanctuary between 306 and 337. It was converted into a baptistery in the fifth century, when the lateral apses were added. Only the mosaics on the roofs of the vaulted aisles are of the same date as the original building. This roof is divided into eight compartments, and there are different designs in each, though only those on the three sets on each side survive; they are in pairs, balancing one another on each side. These mosaics, which consist in the main of scrolls and other diverse motifs shown in isolation against a white ground, are very classical in character; they are virtually floor mosaics transferred to the roof. The mosaics which decorated the central dome have not survived, though there is a sixteenth century painting of them in the Escorial. They included scenes from the Old and New Testaments, bordered below by a river and separated one from another by cariatid figures, not unlike the dividing panels in the Baptistery of the Orthodox at Ravenna. In the apses which terminate the sides of the octagon to the north and south are figural compositions of a rather different character, depicting the "Traditio Legis", where Christ conveys future responsibility for preaching on one side to Peter and on the other to Paul. Our Lord stands in the centre of each apse, with the Apostle before Him, against a background of trees. The mosaics are probably to be assigned to the time of the building's conversion for use as a baptistery in the fifth century. They have, however, been very much restored at subsequent dates, and to-day appear somewhat clumsy. Those in the dome probably belonged to the same date as those in the vaults of the octagon.

The "landscape" backgrounds of the apse mosaics savour of the "picturesque" style, whereas the scrolls that form the decoration of the Chapel of Sts. Rufinus and Secundus in the Lateran are abstract rather than naturalistic, and include birds, lambs and other pieces of Christian symbolism akin to those in the Catacomb paintings. The work must have been done in the early fifth century, probably under Pope Sixtus III (432-440). The decoration of the apse of Sta Maria Maggiore, before its restoration by Torriti in 1295, must also have consisted of scrolls, though to judge by what survives of them, on either side of Torriti's figures of the Coronation of the Virgin, they were much heavier and more stylized than those in the Lateran Baptistry. . .