In the early Middle Ages relations between Church and State were founded on a spirit of co-operation. Three dates are of capital importance. In 380 Theodosius decreed that all his subjects should embrace ‘the faith delivered to the Romans by the Apostle Peter’; in 490 the hierarchy of Gaul baptized Clovis, the young Frankish king, and thereby determined the fate of the barbarian world; while at Christmas in the year 800, Pope St. Leo III conferred the ancient crown of empire upon Charlemagne, a descendant of the invaders. Throughout six hundred years and more, by means of unending courage and endurance, the Church had kept a restraining hand upon those turbulent princes who dominated Europe, with the result that society had returned step by step to the light of civilization.
There was another side to this tremendous achievement. Though herself a spiritual power, the Church had worked well upon the temporal plane; but in doing so she had failed to put first things first. Her leaders had grown deaf to the Gospel precept; by mixing with the world they had lapsed into worldliness. The history of the barbarian epoch of that of continual co-operation between the spiritual and the temporal, a co-operation with Charlemagne treated as a principle of government.
Henri Daniel-Rops, Cathedral and Crusade: Studies of the Medieval Church (1050-1350)