Answer/Explanation:
Church gradually became a defining institution of the Roman Empire. Emperor Constantine issued the Edict of Milan in 313 proclaiming toleration for the Christian religion, and convoked the First Council of Nicaea in 325 whose Nicene Creed included belief in "one, holy, catholic, and apostolic Church". Emperor Theodosius I made Nicene Christianity the state church of the Roman Empire with the Edict of Thessalonian of 380.
After the fall of the Roman Empire in the 5th century, there emerged no single powerful secular government in the West. There was however a central ecclesiastical power in Rome, the Catholic Church. In this power vacuum, the Church rose to become the dominant power in the West. The Church started expanding in the beginning 10th century, and as secular kingdoms gained power at the same time, there naturally arose the conditions for a power struggle between Church and Kingdom over ultimate authority.
In essence, the earliest vision of Christendom was a vision of a Christian theocracy, a government founded upon and upholding Christian values, whose institutions are spread through and over with Christian doctrine. In this period, members of the Christian clergy wield political authority. The specific relationship between the political leaders and the clergy varied but, in theory, the national and political divisions were at times subsumed under the leadership of the Catholic Church as an institution. This model of Church–State relations was accepted by various Church leaders and political leaders in European history.
The classical heritage flourished throughout the Middle Ages in both the Byzantine Greek East and the Latin West. In the Greek philosopher Plato's ideal state there are three major classes, which was representative of the idea of the "tripartite soul", which is expressive of three functions or capacities of the human soul: "reason", "the spirited element", and "appetites" (or "passions"). Will Durant made a convincing case that certain prominent features of Plato's ideal community were discernible in the organization, dogma and effectiveness of "the" Medieval Church in Europe.
