In the early days of the Roman power it was customary to cremate the dead, the ashes being preserved in urns that were ranged in cells known as Columbaria, generally hewn in the living rock. As time went on, however, the Egyptian mode of sepulchre was adopted. Bodies were embalmed and laid in stone or marble coffins which were placed in the basements of tombs of two or more stories, surmounted by round towers with pointed or circular roofs. Of these complex resting-places of the dead the finest now in existence is the Mole or Mausoleum of Hadrian, known as the Castle of S. Angelo, at Rome, which is some 300 feet high and was originally encased in marble. No burial was allowed within the walls of a Roman city, but the approaches were generally lined with tombs as at Rome, at Pompeii, and elsewhere, most of them, though on a smaller scale, of a similar plan to that of Hadrian.
It was in the low, gloomy, dimly lighted subterranean galleries known as catacombs, hewn in the living rock near Rome, that Christian architecture may be said to have had its first crude beginnings. The passages in the walls of which the graves of the dead were hollowed out, widened at intervals into spacious vaulted halls, where the persecuted followers of the crucified Redeemer met in secret for worship or to take part in the funeral services for those they had lost.
It was long taken for granted that it was not until the first issue in A.D. 313 of the Edict of Milan by Constantine, Emperor of the West, and Licinius, Emperor of the East, that the professors of the new faith ventured to erect above ground buildings for the exercise of the rites of their religion, but recent discoveries prove that Christian churches were built as early as the 3rd century in many parts of the Roman empire. To quote but two cases in point, relics of a circular one with a small apse at the eastern end have been found at Antepellius in Asia Minor, and of one of the basilican type at Silchester in England. Moreover, heathen temples were occasionally converted into churches, whilst basilicas were sometimes used for Christian services just as they were.
Some few early Christian churches were possibly modelled on classic tombs such as those referred to in the chapter on Roman architecture, but the more usual form was the basilican, the altar having been placed on the raised platform within the semicircular apse at the eastern end, the bishops and clergy occupying the seats assigned in halls of justice to the prætor and his assessors, whilst the congregation met in the nave and aisles. Ere long, however, to this general plan was added the distinctive feature of transepts or transverse passages running across the entrance to the apse, thus giving to the whole building the form of a cross. Later structural changes were the erection of an arch above the altar, the heightening of the nave, the connecting of the columns between the nave and aisles by arches instead of horizontal architraves, the introduction of windows, to which the collective name of the clerestory or the clear-story was given, in the semicircular heads of the arches and more rarely into the upper part of the low external walls of the aisles, the apse, which was gradually lengthened eastwards, being left comparatively dark, the only light proceeding from the main body of the church. Simultaneously with or in some cases earlier than these alterations, a portico known as the narthex was added at the western end, extending across the whole width of the nave and aisles, for the use of those, such as catechumens or penitents, who were not privileged to enter the church itself. The narthex in its turn was set within an atrium or outer colonnaded court, in the centre of which was a fountain, used by worshippers for ablutions before entering the consecrated building.