From New World Encyclopedia
Unification Aspects:

According to the Divine Principle, God hoped to use the papacy as his instrument to realize the foundation for the Messiah's second coming.

The pope, for his part, stood in a position comparable to the high priest of Israel, who received God's commands through the prophets. He was responsible to lay the spiritual foundation upon which the emperor could realize the ideal Christian state. After crowning and blessing the emperor, the pope was to obey him as one of his subjects in temporal matters. The emperor, in turn, was to lift up and further the spiritual work of the papacy in his realm. (Divine Principle: II.4.3)

Unfortunately, due to the corruption of the clergy and some of the popes themselves, as well as the failures of the kings and emperors, this providence did not work out as planned. "The popes did not lay the spiritual foundation upon which the emperors could stand as the central figures for the foundation of substance." This led to the failure of the Crusades and to the "Babylonian captivity" of the papacy in Avignon. Finally, the failure of the conciliar movement to reform the Catholic Church from within led ultimately to the Protestant Reformation. Since that time, the papacy lost its central position representing the "high priesthood" of Christianity, and consequently Protestantism, with its doctrine of the "priesthood of all believers," came to the fore. The Bible took central place in Christian faith.

Nevertheless, the papacy remains a powerful moral and spiritual force in the world today, upholding the Christian faith and morality which underpin Western civilization.
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teachers and researchers who wish to further pursue these topics from a unification perspective.