Something from the literature: "Early Christian Art and Architecture, works of art and buildings created for the Christian Church in approximately the first 600 years after the time of Christ, and particularly in Italy and the western Mediterranean area. Until the Edict of Milan (313), by which Emperor Constantine the Great made Christianity one of the official religions of the Roman Empire, thereby putting an end to the persecution of the Christians, Christian art was restricted to the decoration of the hidden places of Christian worship, such as catacombs and titulae, private houses used for secret religious meetings.
Under imperial sponsorship, Early Christian architecture flourished throughout the Roman Empire on a monumental scale. Christian religious buildings were of two types, the longitudinal hall, or basilica, and the centralized building, frequently a baptistery or a mausoleum."
All of that is still possible to see in Rome!!! I saw it all, almost.