If one compresses the history of a thousand years into a few paragraphs, one inevitably distorts that history. The pope were left being the only signifiant figure in Rome after the emperors moved out, and of course after the western emperors vanished, at least until the time of Charlemagne. Rome over the centuries changed from a political and economic center to the city of the pope. What happened to Detroit happened to Rome, and almost as quickly. It was/is, not naturally a center of trade or transportation. One of the stupidest things done by the Italians was to rob the pope of his city and make it a national capital. It does not even serve, really, to tie north and South together, for these were and are, really different countries. Milan is a European state, Naples is virtually Africa. IAC. history and the papacy are the only things that kept Rome alive for fifteen hundred years. Otherwise, it would have gone the way of Ephesus and Athens. Rome became the center of the Latin Church. The papal states came into being after the intervention of the Franks, and provided material support for Rome until the 19th century. More than the popes, the leading Roman families were the actual rulers of these states. Cesar Borgia come ver close to making them into a significant kingdom. But the idea fixee in Rome was that the pope has to have this territorial buffer or they could not survive, except as puppets of a powerful king. The displacement of the court to Avignon is evidence. The perceived spiritual power of the papacy, which also gave it political power as a kind of balance wheel, was great, because after Gregory’s time, it was also the center of a great missionary movement. Just as Jerusalem had been to the Jewish diaspora, Rome became to the western Christians, and like the Temple, it gathered money from all over the place. The wealth of the Church was great, like that of the Temple. But again, all this is a matter of historical accident.
Thanks for the concise historical overview, but my response was not intended to encompass the entire period of Rome or the occasions which enabled her secular power, but it helped reveal in what sense and conditions she was always “willing to concede the secular power to the state,” and the contrast btwn her use of the state in being conformable to it, and the NT in doctrine and example.