Among the many religions of oriental origin Christianity was the only one which really became a Roman religion, and any attempt to explain the complex history of the institutional development of the Church of Rome during the early centuries of the empire must start from the analysis of this distinguishing fact, the ‘romanization’ of the Christian Church. A survey of the Roman environment in which Christianity made its first conquests and secured its success is the indispensable preliminary to the study of the process that gradually transformed Christianity in Rome from a religion of foreign groups into the legally established religion of the Roman Empire.