Final answer:
Early Christian and Byzantine churches have plain exteriors due to a focus on interior decoration, reflecting humility before God and the significance of internal space for worship.
Step-by-step explanation:
The plain exteriors of Early Christian and Byzantine churches are primarily a reflection of the emphasis on interior space over the exterior. This focus on the interior correlates with the goals of the congregations and the services held within. Early Christian architecture recognized a need to house large congregations and to separate the faithful from the unfaithful, which led to a shift away from the traditional Roman temple form and towards buildings with significant interior spaces like the basilica. Furthermore, Byzantine churches were also meant to inspire awe, with interiors richly decorated to tell the stories of religious figures and events in a time where most people could not read. Thus, the plain exteriors were results of humility before God and a focus on the richness within, to facilitate worship and to project the power and glory of the Christian God through internal decoration.