"Charlemagne thought highly of Saint Augustine's vision of a Christian kingdom and made it the guiding principle of the realm. Charlemagne's empire unified western and central Europe, bringing stability to lands which had formerly been in turmoil due to massive migrations…. The spiritual kingdom under the papacy and the Kingdom of the Franks under Charlemagne united and formed the Christian empire.
The period of the Christian empire was parallel to the period of the united Kingdom of Israel in the Old Testament Age. In both cases, a monarchic society followed a feudalistic society for the purpose of consolidating a greater sovereignty, population and territory on God's side. It was explained earlier that the pope had been leading the Church from the position of archangel in order to pave the way for an earthly kingdom. But after crowning the emperor and giving him God's blessing, the pope was then to serve him from the position of Cain (cf. Parallels 4). The emperor, in turn, was to uphold the teachings of the papacy and carry on political work to realize a kingdom fit to receive the Messiah. Had they thus built the Christian empire in full accordance with the Will of God, this period would have been the "Last Days of human history," when the Messiah could have come. The new truth would have then appeared to resolve the problems of religion and science as an integrated human endeavor, guiding religion, politics and economy to progress in one unified direction based on God's ideal. On this basis, the foundation for the Second Advent of the Messiah was to have been established. Moreover, with the dawning of the period of the Christian empire, feudalism should have come to a complete end ….
However, the popes and the emperors deviated from the Will of God. This made it impossible for them to realize Charlemagne's founding ideal. As a result, feudalistic society was not dismantled; on the contrary, it grew stronger over the next several centuries. Religion, politics and economy remained divided, with the spiritual kingdom ruled by the papacy coming into frequent conflict with earthly kingdoms ruled by kings.
The Christian empire failed to build a unified kingdom to which the Messiah could come. Charlemagne built his empire when the foundation of early feudal society was ripe for consolidation into a strong monarchy. However, he never fully subjugated the vested powers of the feudal lords. Instead, the feudal system strengthened, with the Holy Roman Emperor reduced to just one of the great feudal lords." (Exposition of the Divine Principle, 1996, 337, 338)
"Jesus said, "You know that the rulers of the gentiles lord it over them, and their great men exercise authority over them. It shall not be so among you; but whoever would be great among you must be your servant, and whoever would be first among you must be your slave; even as the Son of man came not to be served but to serve, and to give his life as a ransom for many." (Matthew 20:25-28)
"Let no one seek his own good, but the good of his neighbor." (1 Corinthians 10:24)
Charlemagne's coronation as Holy Roman Emperor is regarded as a providential event. The period from his reign through until the end of the Carolingian dynasty represents a 120-year parallel period to that of the United Kingdom in Israel under David and Solomon. Both 120-year periods are followed by four hundred years of division. It is believed that God had intended, under the Holy Roman Empire, to establish a foundation for the Kingdom. Charlemagne's coronation represented a reversal of external dominion over internal, and put the spiritual as the subject position. Thus, 'the spiritual kingdom under the papacy and the Kingdom of the Franks under Charlemagne united and formed the Christian empire' (Exposition of the Divine Principle, 1996, 339).
The idea to unite entire Christendom into single political organism where religion and governance are harmoniously unified to serve the Lord itself sounds very promising should it be accomplished properly. Should kings and popes deny their selfish ways and seek no personal gain, be it power, land or people, and search to govern in accordance with Lord's teaching, the outcome of the Holy Roman Empire would be much more successful both politically and in the terms of lives of countless peoples and nations comprising it. Surely, there would be overall less bloodshed due to feuds and crimes, and more prosperous economy able to send and support numerous missions around Europe and further around the world.
Ideally, as the Vicar of Christ, the pope would have lead society spiritually while the Emperor carried on 'political work to realize a kingdom to receive the Messiah'. It was under the German princes that the Holy Roman Empire reached its greatest extent, and achieved most influence. However, the ideal of a partnership between the papacy and the temporal authority never really worked. Often, there was rivalry between these two institutions; nor was the Emperor's authority recognized outside the territory which he actually ruled. Officially Christian, the kings of the Holy Roman Empire were descendants of Germanic tribes, continuing traditional gentile style of governance, each fighting to preserve his own domain and "exercise authority" over others. In Unification thought, the medieval world under the Holy Roman Empire could have established a foundation for the construction of God's kingdom, but it failed to unify people as it also failed to hold the spiritual and the political (internal and external) in harmony. Thus, this experiment failed and as the medieval world, with its Holy Roman Empire, declined, to be replaced by the Westphalian system of sovereign nation states, democracy and religious freedom replaced monarchy and limited or no freedom of belief.