The first post in this series can be found here.
It's remarkable to realize that the domed church came late to Rome. The city famous for St. Peter's and it's many domed churches did not see any domed construction after the 5th century until the 16th century. All the major developments in domed church architecture took place in the Greek speaking Eastern Roman Empire, or as we know it, the Byzantine Empire, and didn't find their way to the city of Rome until Bramante built the Tempietto. While major inventions such as the dome resting on four arches and the dome clerestory took place in the Christian East, the city of Rome planted the seeds of those ideas in some of its earliest surviving Christian structures and churches.
The ceiling of the cubiculum in the Catacombs of St. Marcellinus and St. Peter.
We hardly recognize this ceiling painting as Christian at all. None of the familiar symbols and images of Christianity appear anywhere, no crosses, no images of Christ, no Virgin Mary, no saints. At first glance, this ceiling painting does not look much different from any other tomb imagery of the period.
In the center medallion we see The Good Shepherd, but this would not automatically identify this as a Christian burial. He could be Apollo Protector of Flocks, or Mercury another Protector of Flocks and a Bearer of Souls to the underworld. The story told in four panels between praying figures is the story of Jonah. In the undamaged panels we see Jonah thrown overboard and swallowed by the whale. The whale spits up Jonah on the beach before Nineveh, and finally there is Jonah resting under the gourd vine.
It's possible that the artists employed in painting these tombs may not have been Christians themselves. They were maybe second or third string house painters hired for a another tomb job for another mystery religion. They borrowed heavily from what they already knew, the stories from Classical mythology. Jonah under the gourd vine looks like Bacchus under a grape vine arbor.
Early Christian architecture -- at least what survives -- does not appear until after the Emperor Constantine ended Christian persecution with the Edict of Milan in 313 and afterwords gave the religion state patronage (though Christianity would not become the state religion of the Roman Empire until the Edict of Thessalonica under Emperor Theodosius in 380).
Most of the early churches built by Constantine and his successors were adaptations of the Roman basilica, originally secular government buildings with a long center hall lit by clerestory windows and flanked by columned aisles. Much less common were round churches built for the ritual of circumambulation, walking around a holy site, usually a saint's tomb, always keeping it to your right. In ancient times all religious people in the Mediterranean world practiced circumambulation. Today it mostly survives in Islam. Early Christians circumambulated, and round churches were built for that ritual.
A round church built for circumambulation would seem to be a natural for a domed ceiling. Not necessarily.
The Church of Santa Costanza may well be the last dome built in ancient Rome. The city that pioneered domed architecture would not see another dome built within its walls until the 16th century.
In the meantime, ambitious and inventive domed architecture moved far to the east in the Byzantine Empire and later in Islam.