After the edict of Milan of 313 promulgated by Constantine the Great (313–337), the Roman Empire allowed the worship of the Christian religion. From then on, Christians used the basilica architectural typology for the construction of new temples.
A Christian basilica itself in the architectural sense is understood as any rectangular plan with one or more apses in the front "Testero (architecture)") and with naves "Nave (architecture)") along its length separated by columns "Column (architecture)") (or pilasters), on which their corresponding Roman-type arches or architraves rest. The aforementioned naves (usually three) end in the apse. In the apse the altar is placed and the officiants are arranged around it. In front, in the presbytery "Presbytery (architecture)"), the priests are located, while the faithful occupy the rest of the nave or naves.
The elementary basilica plan consists of longitudinal naves without transepts. But many basilicas have a transept. Frequently the transept hardly highlights the sides of the nave. Early Christian basilicas did not have an accentuated transept. Although initially Christian temples followed the construction guidelines of basilicas, they soon gave way to other forms, such as the Latin cross or Greek cross plan, which became widespread without the basilica form disappearing.
The roof of the naves usually consists of an artistically decorated wooden framework visible from the inside or hidden by a coffered ceiling: sometimes, they have a vault in the side naves and the apse always ends in a quarter-sphere vault.
The lighting of the basilicas is obtained through open windows in the upper part (cleristory) of the central nave, higher than the side ones, and through other windows that are located in the apse and in the "Fronton (architecture)" front of the building. All of them used to be closed with sheets of perforated or openwork marble to let in light and prevent the action of destructive elements. But transparent sheets of unperforated alabaster and even stained glass were also used in sumptuous basilicas, as inferred from some texts by Saint John Chrysostom and Prudentius.
The interior decoration is achieved by the same architectural lines of the building with its classic moldings and by different decorations of paintings and mosaics, especially on the upper wall of the triumphal arch and in the always magnificently decorated apses.
Frequently, the basilicas were arranged oriented along the main axis of the nave so that the apse faced west. But since the century, the Byzantine churches, setting an example, were oriented in the opposite direction since the priest (who when offering the sacrifice looked to the East) no longer celebrated facing the people as before.
In addition to the basilica-type churches, in this first period of the Constantinian peace there were other smaller ones, with a simply rectangular or square or even round plan, which served as oratories "Oratory (religion)") or sepulchral chapels or memorials of the martyrs (cellae memoriae) and there were no shortage of others of polygonal or circular shape intended for baptisteries. All of them and even almost all the great basilicas were built from the ground up and only a small number of them had previously been public buildings or temples that were enabled for Catholic worship.
The interior distribution of the basilicas in the first centuries of peace, following the Constantinian model, is as follows:
At the entrance to the presbytery, as if to isolate it from the rest of the church, columns stood that supported a marble or wooden architrave to fix votive offerings and lamps on it. This architectural complex is called pergula and corresponds to the iconostasis of the eastern churches, which is a more closed and complete body and is decorated with a multitude of devotional images.
In some basilicas, on part of the side naves, there was a floor with galleries that overlooked the central nave and were generally reserved for virgins and widows. This place was called gynnaeceum. The ground floor of the left nave (that is, the Gospel) was dedicated to women and was called matronikion. On the right, for the men, it was called andron and each group entered the basilica through its corresponding door. The middle one, which was called argéntea and speciosa, served as the entrance for the clerics. On the right side and separated by a fence or parapet, the men of distinction were placed and the matrons were on the left. Hence the names senatorium and matronaeum that were given to them, respectively.
On the sides of the choir there were wide pulpits or pulpits for reading the Gospel and the Epistle. The placement of the faithful was as follows:
This genre of basilica architecture continued to prevail in Rome and its surroundings quite firmly and exclusively until modern times (with rare exceptions) but in the other regions of the Catholic world it evolved much later, transforming into different genres and styles.