Today I visited Hadrian’s ‘Villa’ near Tivoli — an easily bussable distance from Rome. As I walked around, one of the many thoughts that struck me was the similarities between Roman architecture and early Christian architecture — the architecture of the West to the end of Romanesque (which simply became Renaissance in Italy, consciously looking for classical models) and the East for a lot longer.
It is my contention that these architectural styles are not borrowed from ancient ‘pagan’ cult but from domestic, public, and commercial architecture first and foremost. Ancient Christians were very wary of their polytheist neighbours, their spaces, and their cultic practices well beyond Constantine (contrary to what some people will try to tell you).

To return to Hadrian’s Villa, then. Many rooms and buildings at Hadrian’s Villa come fully equipped with apses — semi-cylindrical ends of rooms with semi-domes at their tops. The apse is a common feature of most churches within a certain period. It persists into the Gothic and beyond. Sant’Agnese, a seventh-century basilica in Rome, has a nice apse.
And that word — basilica. This is a Roman word, and not one related to religio. In contemporary church-building lingo, it refers to a Roman Catholilc church that has been all blessed up by the Pope. In ancient Rome, it referred to a large, covered building used for conducted business, originally based on a building seen by M Porcius Cato the Elder whilst abroad called a basilike — a royal building.
The basilica was a lawcourt and place of commerce and enterprise, not a temple. Also, it had an apse where the magistrates would sit to listen to people’s entreaties. This, in fact, is not unlike the original function of a Christian apse, where the bishop would sit in the middle flanked by his presbyters — today, you are likely only to see a painting of concelebrating bishops.
Besides having an apse, the floor plan of a basilica, such as the fourth-century Basilica of Maxentius, Rome, demonstrates the indebtedness of the Christian basilica to its secular forebear.
In the fourth century, Christians begin building churches in the form of basilicas, apses and all. The earliest, Basilica Constantiana, is now San Giovanni in Laterano — technically Rome’s cathedral. I’ve not visited it, but it was built in the fourth century under Constantine; much of that early architecture, I understand, is obscured by its mediaeval redecorating. The other major Roman basilica of the period is Old St Peter’s, remains of which are visible beneath — you guessed it — St Peter’s.* These Roman basilicas set the stage for centuries of basilica building to come.
Why do Christians adopt this form? Why do they take on this and many other features of ancient Roman architecture — columns and round arches and domes and niches for statues and on and on? Is it that wicked heresy of ‘Constantinianism’ destroying the pure churches that once met in houses?
Well, given the frescoes at Dura Europos and the Catacombs, as well as the sheer size of some of these ‘house’ churches, I do not think that they went reluctantly to basilicas.
The basilica was adopted because it is eminently practical. It is suited to Christian liturgical functions — basically as we know them since at least the late first century. The apse, besides any practical function, helps set out that side of the building as a focus. Adding a mosaic only helps — as with all the other embellishments, such as fancy capitals.
Just as Augustine put his secular rhetorical training into Christian service, so the architects of the age put their secular architectural training into Christian service.
*Maybe Constantinian, maybe Constantian. I have no dog in that fight.