This article discusses the origins and characteristics of the Greek Orthodox churches of Asia Minor in the context of ethnic diversity. It is customary to call Asia Minor or Anatolia the 'cradle of civilisations'. Although this may seem exaggerated, the existence of three empires - the Roman, Byzantine and Ottoman make it special, at least in the field of architecture. The case study area is the city of Kayseri in Central Anatolia. In antiquity this was Caesarea, the capital of Cappadocia region, which played an important role in Christianity during the Roman period. Although it is known that the origins of its underground settlements are earlier, the volcanic tufa structure of the region assisted the creation of secret holy places and the dwellings of early Christians. During the Byzantine Empire, Kayseri was a religious centre and owing to the ethnic composition of the Ottoman Empire this continued until the twentieth century without interruption. Among the three sample churches examined, the one in Zincidere-Kayseri was constructed over an earlier underground church. The attitudes of the Ottoman Sultanate and its approach to church architecture can be seen in the evolution of the construction and decorative details. Within this context the local ethnic composition of Ottoman society is examined, giving insights into the believers in two International religions.