The architecture of the temple
The building presents an accurate construction technique, even though not of particular value, with outer walls in opus mixtum with no particular regularity.
The outside features a rhythmic series of arches outlining the simple cubic volumes of the building: the high main body with a square plan and the lower rectangular presbytery, with its straight back wall. But it is above all on the inside that the architectural solutions define and surprisingly dilate the space, giving a sense of airiness to volumes that are not particularly large. The vertical impetus of the main body, underlined by niches with arches on the walls and concluded at the top by a groin vault, recalls the settings with a central dome of Christian architecture, while the presbytery, with three aisles, appears to be the natural extension of the main body, onto which it opens with a monumental triple arch in brick, supported by marble columns, outlining the eastern front. This aspect can be appreciated by viewing it from the main entrance: a broad portal outlined by a large arch, built into the western façade since the origin, along which an outer hall was probably added only at a later date. At present the Temple is accessed from a side door, opened in the southern perimeter since the origin. It is reached by passing through an area erected in the Middle Ages, presumably acting as a sacristy. From the small door in the south side, slightly altered in the Middle Ages, you enter the presbytery where you can admire the singular configuration of the area where the altar stood: situated in front of the back wall in the centre of which is a niche with an arch, similar to those in the main body. While the division of the eastern zone derives from a consolidated tradition for Christian places of worship, the result obtained here is certainly unique as regards the choice of architecture and construction methods, thanks to the creation of three aisles covered by barrel vaults resting on a system of a pair of columns and pillars that support marble shelves and architraves of the Roman period. Decorated with plant motifs, these elements are used in the early medieval building with a clear intent to refer to antiquity. There are also reclaimed columns, pillars and elements of the Byzantine period in the partition that separates the presbytery from the main body of the church. All made of Greek marble, they were recovered from older buildings. Instead the bases and capitals were created for the occasion in the early Middle Ages; they are similar to those of the canopy over the baptismal font of the patriarch Callixtus, carved between the fourth and the fifth decade of the eighth century to adorn the baptistery of the cathedral and now to be seen in the Christian Museum at the Cathedral. By the same hand or at least of the same early medieval period are the small capitals placed on the pillars of the presbytery enclosure to support the wooden architrave of the iconostasis, perhaps also original, which in the Middle Ages held the wood statues of Saint John and Our Lady standing next to a cross.
