Gregory the Great [circa 540 AD to 604], was one of the most dynamic men of the Middle Ages. Elected to the Papal office in 590 AD, it was Gregory who largely took the remnants of Roman civilization and its citizenry and during a time of tumult and change; assumed responsibility for the social and moral welfare of the Christian state. This practical management of society during difficult times was important in the development of a mixture of ecclesiastical and state power under the auspices of Rome.
However, as with most 'enlightenment' dogma, the Church never did take over the 'state'. The state however, did indeed take over the Church, or at the very least, directed its prestige towards secular objectives. We see this beginning with the reign of Charlemagne and proceeding down through the Ottonians in the 11th century; the French and Spanish from 1300-1500; and the rich Italian aristocracy including the Medicis from 1400-1650. By the late Middle Ages, Popes, Bishoprics and Diocesans were bought and sold by secular powers, entirely for political purposes.
Rome did not fall, it was taken over and transformed into Germanic kingdoms. There was no 'fall' of civilization into insufferably dense darkness. The Roman civilization was in many ways a deterrent to the improvement of man, life and society. Technology, the arts, innovation, all suffered, as did the 90% of Romans who were impoverished or enslaved.
What Gregory and others within the Church had to do, was to pick up the relevant pieces of the Roman world, and merge them with a new social order premised around the only authority left to manage the transformation; namely, the Church. In Roman cities every aspect of life, from water provision to welfare and education, now fell under the rubric of the organized and responsive Church. Without the Church, there would have indeed been a lot more chaos, death, and suffering post the decline of the rotted Roman empire.
Gregory was in many ways the savior of social and civilizational order. The Papacy repaired, invested, and improved upon the existing infrastructure left by the Romans. There was never a descent into barbarity. What is dark is our inability to read through the thousands of tonnes of documents in Latin, which are available, and which describe the various works, efforts and attempts to recreate a new and better social order. Indeed there is little doubt that the Lombardian and Visigothic Kingdoms were richer, better organized and more literate than the Roman. In the case of the latter, it proved to be a magnet for the Moslem Jihad. Apparently Jihad is more attracted to wealth, coin and beauty.
Gregory's main intellectual achievement was to bring Augustinian theology to the masses. He wrote and expounded on Augustine's early 5th century theology which was Platonist in its construction. Thus we have the merging into Christianity of Plato's doctrines around the soul, eternal life, the dichotomy of the body and soul; and the moral life. St. Thomas Acquinas Christianized Aristotle. Augustine and Gregory Christianized Plato. There was therefore, no conflict whatsoever between Christianity and the ancient pagan Philosophers.
See also C. Straw, 'Gregory the Great', Berkeley 1988.