One of the largest consequences of the collapse of the Roman Empire in western Europe was the virtually independent development of the Christian church. The position of the first influencial Pope may help to understand why papal authority got so powerful over the years.
The first great climax of papal authority development in the west was reached under Pope Gregory I (590-604), known as Gregory the Great. More consistently than any of his predecessors, he sought to extend papal authority over the Christian Church. He had greater success in the missionary efforts he encouraged, particularly those of St. Augustine of Canterbury in England, than in interfering with well-established churches under strong kings, such as the Church of Spain and the Church of Gaul. But his most lasting work was in Italy itself. During his pontificate the conversion of Arian Lombards was undertaken and the city of Rome was transformed into a political and administrative unit, which was the beginning of the Papal States.
This development was to give the papacy a territorial interest and a political role that would never be fully divorced from its spiritual role and that often, it was claimed for centuries and centuries thereafter, would get in the way of that spiritual role. Yet this was not how the problem appeared to Gregory himself and his contemporaries, since they simply had to defend Rome from people like the Lombards. The pope had to provide secure government and food for his citizens.
Moreover, if Gregory the Great was to function as the independent head of the church, he had to preserve his political independence. Otherwise, he would become as dependent on local autorities as the patriarch of Constinantople or the bishops of Spain and Gaul.
One and a half century later, Gregory's successors had no alternative than to call, unwillingly, on the Franks for protection from the Lombards. That sequel made it clear beyond a doubt that the papacy needed an independent political base if it was to make good its spiritual claims. The history of the next thousand years, not excluding the experience of the Protestant churches after the Reformation, did nothing to show this assumption to have been false.