The first monumental churches, built after Constantine legalised Christianity in 313: long timber-roofed basilicas with a tall clerestoried nave, lower aisles and an apse, entered through an atrium and narthex; and round or polygonal buildings for baptism and for the tombs of martyrs.
Before 313 Christians met in houses; after Constantine’s Edict of Milan the emperor himself became the church’s greatest patron. For the new congregations the pagan temple (a house for a statue, entered only by priests) was useless. The model was instead the Roman basilica, the great civic hall of the forum: a long room divided by colonnades into a wide central nave and lower side aisles, lit by windows in the upper walls of the nave (the clerestory), with an apse at one end where a magistrate had sat. Constantine’s Lateran basilica (begun c. 313) and Old St Peter’s (begun c. 320s) fixed the type.
The Christian basilica is organised as a path. From the street one enters a colonnaded forecourt, the atrium, often with a fountain for washing; then a porch or vestibule, the narthex; then the nave, whose colonnades draw the eye and the procession toward the altar before the apse, framed by a great arch that came to be called the triumphal arch. The largest churches, such as Old St Peter’s, had five aisles and a transverse hall, the transept, before the apse.
Construction was light and quick. Walls were of brick-faced concrete, columns and capitals were often reused (spolia) from older buildings, and the nave was spanned not by a vault but by open timber trusses, which let wide naves rest on thin walls carried by colonnades or arcades. The contrast between exterior and interior was deliberate: outside, plain brick; inside, marble revetment, gilded ceilings and mosaics glowing in the apse and along the nave walls, as they still do at Santa Maria Maggiore and at Sant’Apollinare Nuovo and Sant’Apollinare in Classe in Ravenna.
Alongside the basilica stood the centralised building (circular, octagonal or cross-shaped), inherited from Roman mausolea and bath halls. It served for baptisteries, where the whole community gathered round the font, and for martyria and imperial tombs such as Santa Costanza in Rome (c. 350), a rotunda whose dome rests on a ring of paired columns within a vaulted ambulatory. The two traditions, the longitudinal hall and the domed centre, would be fused by the Byzantine builders of the sixth century.