Since Jesus famously took out a coin and said “render unto Caesar that which is Caesar’s and unto God that which is God’s” the Church and State have been separated. As St. Augustine famously put it, there is a city of God and a city of Man. The Middle Ages illustrated that division more than most eras; with the relative weakness of political rulers the Catholic Church exercised considerable authority on a temporal level. That authority proved too great for the invading Islamic armies, who never were able to gain a foothold in Western Europe, while in the East, where the successor to the Roman Empire held the Church in a vassal state, Islam flourished and eventually drove the Orthodox Churches out. The bifurcation of power in the West made them strong, able to resist the invasion of the resurrected Arian and Manichean heresy that was Islam. And when the Catholic Church grew too chummy with the political powers there was a rebellion (the Protestant Reformation) which fractured the ability of the ruling class to use religion as a tool of societal domination.