It was this kind of plan that architects adopted when Constantine officially recognized the church in 313 and was converted to Christianity. Whether in Rome or in other cities of the empire, Constantine’s architects took their inspiration not from pagan temples, old-fashioned in the 4th century, but, rather, from a secular building type of utilitarian character, the basilica, which had served as a hall of assembly, commerce, reception, or lawmaking. Of Hellenistic origin both in form and in name (stoa basileōs, or “royal room”), the basilica under the Romans varied in plan and size according to use and to the importance of the social group to which it belonged. It could be either a simple hall or one divided by columns into three longitudinal aisles, or surrounded on three sides by one- or two-story arcades with a dais on a short or long side.

The first Christian basilicas, built in Rome, were variations of secular basilicas adapted to the new cult. St. John Lateran, superficially transformed in the 17th and 19th centuries, is the oldest, begun about 313. It was followed by St. Peter’s (replaced in the 16th century by the present church) in the last years of the reign of Constantine and his sons. San Clemente, Santa Pudenziana, St. Paul’s Outside the Walls, San Sebastiano, Santa Sabina, and others belong to the late 4th and to the 5th century.

These were, in most cases, halls with five longitudinal aisles, the central one raised and lit directly by windows piercing the high walls. A semicircular protrusion of the wall, or apse, on the short side opposite the entrance, first on the west side but later on the east side, was covered over with a half-dome vault, while the so-called hall itself usually had a wooden frame roof. The side aisles were separated from the central nave by rows of columns. These supported a rectilinear entablature, or solid horizontal section; later this entablature was replaced by a series of arches resting on capitals (San Clemente, 360; St. Paul Outside the Walls, 385; and Santa Sabina, 422–432), but at Santa Maria Maggiore, still intact under its 17th-century Baroque facing, Pope Sixtus III (432–440) returned to the Classical form of the straight entablature.

Whereas St. John Lateran and San Clemente were used entirely for the eucharistic service, St. Peter’s and St. Paul Outside the Walls were also martyria, buildings commemorating the martyrdom of their titular saints. This commemorative function influenced the plan: in front of the apse a large transept, sheltering the site of the relics, facilitated the presentation of the liturgy and the circulation of the faithful around the sacred place.

The churches built under Constantine at Constantinople and in Palestine were more complex in plan and structure. The destroyed church of the Holy Apostles at Constantinople, known only through a description by Eusebius of Caesarea, was begun in 333 and completed by Constantius II (337–361). It was cross-shaped, and a drum—a cylindrical or polygonal wall that usually supports a dome—rose above the crossing, probably covered by a conical wooden roof. The sarcophagus of Constantine stood in the centre, surrounded by memorials to the 12 Apostles. It was less a church than a mausoleum in honour of the first Christian emperor, who was made to appear in that church as the 13th Apostle. In the sanctuaries in Jerusalem and in the church of Bethlehem, the commemorative building and the hall of worship (basilica) were united. At Jerusalem several structures combine to form the church of the Holy Sepulchre. The Anastasis (the Resurrection), a rotunda approximately 131 feet (40 metres) in diameter whose foundations and remains of the walls have been discovered under later additions, was built about 340 on the “tomb” of Christ, the funeral place hewn into the rock and surmounted by a small temple. Two levels of galleries surrounded the rotunda, and the whole was covered by a wooden cupola. The site of Golgotha, open to the sky, was preceded toward the east by a martyrium, a five-aisled basilica with tribunes, or raised platforms, intended for gatherings of the faithful. It probably terminated at the west with a rotunda, open to the nave and surrounded by 12 columns marking the place where it was thought that Helena, mother of Constantine, had found Christ’s cross. The church of the Holy Sepulchre and the court of Golgotha were surrounded by porches aligned along the exterior aisles of the basilica (martyrium) and on the galleries of an atrium (forecourt), which preceded the martyrium. The Constantinian basilica at Bethlehem (still partly intact) with five aisles without tribunes and the central aisle raised, was preceded on the west by an atrium and terminated in the east by an octagon built above the Grotto of the Nativity. Justinian had it replaced by a triconch (three-apsed building), which still stands.

The considerable dimensions of all these complexes (657 by 230 feet [200 by 70 metres] including the atrium in the case of St. Peter’s, the largest) and the richness of their decoration (marble columns and inlay; mosaics on the vaults; opus sectile, or inlaid stone, in the pavements; and sometimes gold coffered ceilings, as at the Martyrium in Jerusalem) lent to these structures a sumptuousness that made them the equals in scale and splendour of the imperial palaces. Moreover, the liturgical ceremony became progressively assimilated with that of the court; the image of Christ standing or enthroned in the apses of the basilicas evoked the emperor presiding over reception ceremonies in his palace.

The differences of plan between the Greek and the Latin Constantinian churches have been a subject of controversy. One hypothesis explains them on the grounds of the Greek traditions—differing from those of the Romans—of the cult of heroes and of the dead. Since the churches of Palestine commemorate the earthly appearances of Christ, his Birth, and his Resurrection, they are comparable in function to the Greek hērōa, which are commemorative temples. In the West, however, the churches enclose actual graves, the focus of a funerary cult, completely different from the cult of the Greek heroes.

The central-plan building, round, polygonal, or cruciform in design, gathered considerable momentum in the West as well as in the East in the course of the 4th and 5th centuries. The deconsecrated church of Santa Costanza in Rome, built between 337 and 350 for members of the imperial family, was a rotunda with an ambulatory or circular walkway separated from the central area by columns; the mausoleum of Centcelles (Tarragona) in Spain, likewise a rotunda, was probably the burial place of Constans, son of Constantine, assassinated in 350. Both are somewhat related in type to the Tomb of Diocletian (c. 300) at Split in Croatia.

Milan, which had been the imperial residence several times since 350 and seat of the bishop St. Ambrose since 374, has preserved the remains of some centrally planned churches of the 4th century. San Lorenzo Maggiore, begun about 370, is a quadrifoil room with four niches and ambulatory; an octagon adjoining it (today Sant’Aquilino) was formerly an imperial mausoleum or baptistery. The church of the Holy Apostles, the present San Nazaro Maggiore (begun in 382), is cruciform in plan with an apse in the east, built in imitation of the church of the same name at Constantinople. At Cologne, the oval plan of St. Gereon (built about 380) is enriched by eight smaller apses (apsidioles), an apse in the east, and a narthex, or large rectangular room placed before the west entrance. At Antioch in Syria an octagonal structure, called the Golden House because of its gilded roofing, was begun as early as 327 by Constantine. Near this city at Kaussich are preserved the foundations of a cruciform church, built between 378 and the end of the 4th century; it served both the normal cult and the commemoration of three martyrs whose sarcophagi were found in the transept.

Since most of these churches were not consecrated to the cult of a martyr, the mausoleum could not be the only source of inspiration for their plans. Imperial reception rooms may have been models. The octagonal church at Antioch and that of San Lorenzo in Milan were probably palace churches, and the transfer of plan from one to the other seems perfectly plausible.

In the 5th century, as local schools formed, the unity of Christian architecture with that of the empire ceased to exist. In Italy, although basilicas of the Classical type continued to be built, they assimilated Eastern influences. North Africa modified the basilica plan only by multiplying the number of side aisles (Damous-el-Karita in Carthage has eight) or by adding apses.

The ecclesiastical architecture of the East is more varied, partly as a result of differences in the liturgies. The monastery of St. John the Baptist of Studium in Constantinople (463) and the church of the Acheiropoietos at Thessalonica (470) were basilicas with tribunes and narthexes, which, in their proportions, approached those of centrally planned structures. The large central aisle, inaccessible to the faithful, was reserved for the service of the eucharist, the side aisles for the men, the tribunes for the women, and the narthex for those who for some reason could not participate in the communion. Later, this form of basilica spread into the Greek countries and gave birth to an essentially Byzantine type of church, the “domed basilica.” Nevertheless, the ordinary basilica plan, of three or five aisles with an apse in the east, remained no less popular in Greece, on the islands, in the Balkans (Stobi in Serbia), and in the Middle East. In Asia Minor the church of St. John of Ephesus (450), later replaced by a building of Justinian, was cruciform in plan, inspired by the church of the Holy Apostles in Constantinople. Extending from the square ciborium (a canopy over the altar supported by columns) placed at the crossing were four wings, the eastern one having an apse and five aisles, the others having three aisles.

In Syria, Israel, and Jordan a particularly large number of 5th-, 6th-, and 7th-century churches are preserved. The triple influence of the Greek countries, Constantinople, and the imperial sanctuaries of the Holy Land resulted in many plans, whereas the materials and construction methods remained in the tradition of the region. At Qalʿat as-Simʿān near Aleppo, Syria, lies the ruin of a martyrium built about 470 around the column on which the ascetic St. Simeon Stylites spent the last years of his life. The precious relic was enclosed by a central octagon of considerable dimensions, adjoined by four arms of a cross in the form of basilicas. At Jarash in Jordan the church of the Apostles and Martyrs (465) is a cross inscribed in a square, heralding a typically Byzantine plan of later centuries. Also at Jarash, the triple church dedicated to Saints Cosmas and Damian, to St. John the Baptist, and to St. George consists of two basilicas flanking a rotunda with an ambulatory (528–533) and an apse in the east. Other rotundas are at Buṣrā ash-Shām and Izraʿ in Syria, as well as the octagonal church of the Theotokos (484) on Mount Gerizim (now in the occupied West Bank territory). The sanctuaries of Egypt were also influenced by those of Constantinople, the Greek countries, and Italy. The cathedral of Hermopolis (al-Ashmūnayn), built about 430–440 in southern Egypt, and the martyrium-church of St. Menas, nearer the coast (first half of the 5th century), combine the influences of Constantinople and Italy in the three-lobed sanctuary, the transept, and the long basilica room with three aisles. The ties between the Latin West and the Greek East, particularly strong in the 4th and 5th centuries, relaxed in the 6th. Beginning with the reign of Justinian, a true Byzantine architecture developed from the new capital. In the western Mediterranean, the end of the ancient world and of early Christian architecture came with the fall of the Roman Empire in 476.