In his De Civitate Dei, Augustine defined the Church and the Empire as ‘Two Cities’: one heavenly, one earthly. In his work, comparisons between pagan Rome and the Church are constant. Opposing the old dying pagan world that holds on to the glory of its past lies the future of the new city, the City of God, that is, the Church. We do not intend to analyse here the work of Augustine. But it certainly proves very interesting to see how a duality is established, a symmetrical comparison between the two entities, as if the two were identical structures. Other patristic writings also refer constantly to this comparison between equals.

Actually, early Christians, at the very beginning, were aware of the need of an organization. But it was not be until the time of Constantine that the ‘charismatic Church’, invested with auctoritas, turned into a well-organized institution, covering the entire State and where one of its main features was the potestas of their leaders, at both a local and a regional level. This unheard of authority at a small and medium scale had been received directly from a civil power, the Empire, and ceded in Constantine's era. Soon, and on a number of occasions, the new Church challenged the emperors (the episode of Ambrose and Theodosius leaves little room for doubt), and rapidly increased its influence when the Roman State disintegrated. As we can see in the West, bishops, supported by the local aristocracy, became leaders (religious, economic, political, and even military!) in their urban communities, also providing cohesion to extensive territories that shared the same episcopal administration, created by Rome a century earlier. The vacuum left behind by the State not only meant an increase in their power, but they even ended up becoming the authorized spokespersons who interacted with the new civil and military powers, which had now become changing groups of barbarians bent on forging local entities that would enable them to obtain the stability that Rome denied them. In the end, the Church managed to reach a fruitful understanding with the invaders, so much so that what we could call ‘state churches’ linked to each new barbarian kingdoms began to spring up.