It was the Age of Faith, not the Age of Reason. Fear, uncertainty, and awe of the unknown defined Medieval society. This was a culture that had, since the fifth century, combined the remnants of the Roman Empire, the masses of (mostly German) tribal people, and the vigorous (and still relatively new) Catholic religion to produce what would eventually be a culture that covered the globe. In a curious way, the Roman, tribal, and Catholic elements all mixed together, each contributing to the culture in its own unique way. The Roman Empire left many useful items in its wake. The Latin language , while no longer used by anyone to raise their children, had become the international language of diplomacy, learning, and, most importantly, religion. Over a century before the Rome fell, Catholicism became the official religion of the empire. This had a greater impact on the church than it did on the empire.
When the Catholic (or simply "Christian," as in "followers of Christ") Church became the state religion of Rome in the 4th century, it adopted the governing techniques of the Romans enthusiastically. For now the followers of Christ had the full weight of the mighty Roman Empire behind them. The Roman form of government lasted over a thousand years because it was efficient and the Church fathers knew a good thing when they saw it. When the Empire in the west fell in the 5th century, the Catholic Church continued as a Roman institution. While everyone else spoke a babble of new languages and Latin dialects, the church did its business in Latin. The pope's official titles was "Pontifex Maximus ," the same title the pagan Romans had always given their chief priest. Many of the terms and titles used in the church administration came directly from Roman practice.
The barbarians who overthrew Rome basically trashed the Roman administrative structure. They soon realized that all those Roman bureaucrats did serve some useful purposes and began to rebuild society using Roman governing techniques. Most of the titles of nobility are derived from the titles Roman administrators were using when the empire fell. Count, Duke, and Captal are all direct derivatives of Roman bureaucratic titles (Comes, Dux, and Capitalis). The Roman practice of dividing territory into provinces and districts was also borrowed. Sometimes the provincial and district borders remained the same.
The Roman Empire did not fall all at once, the Greek speaking east enduring until 1453, when the Turks took Constantinople . The city had been named after the 4th century Roman emperor, Constantine, who made Christianity a recognized religion (and encouraged its spread). But even as Constantine accepted Christianity and reorganized the empire, the barbarian tribes were closing in. Rome ceased to be the capital of the western half of the empire in the 5th century, when barbarian armies overran Italy. Roman armies from the eastern half of the empire were successful in recapturing many of the western provinces in the 6th and 7th centuries, but in the 7th and 8th centuries the new Moslem religion, and its powerful armies, appeared. This put an end to eastern Roman (Byzantine) attempts to reunite the empire. All that was left in the west was the Christian church, and its administration modeled, after the Roman model.
For several centuries the church had its hands full dealing with the barbarians, many of whom where enthusiastically pagan and loved to torment the Christians. But eventually the faith (not to mention better organization and determination) of the Christian church prevailed. Between the 6th and 10th centuries, most of the major barbarian rulers and their people became Christian.
At one point, in the 8th century, a curious event occurred. There arose some interest in reviving the Roman empire, or at least the title of emperor. What was especially curious about this was that it was the use of absolute rule by an emperor that had contributed much to the failure of the empire.. What had kept the Romans going for so long was their system of government, a quasi-democracy (local autonomy, within a more or less absolute monarchy). It was only in the last two centuries of the Roman empire that they had gotten away from this mixed form of government, going over to an absolute monarchy at all levels, and that was a major contributor to the empire's demise. But the barbarian kings were quite taken with the idea of becoming the rulers in the Roman mode. So, in 800 AD, the pope crowned the unsuspecting Charles the Great, King of the Franks (a German tribe) as the "Holy Roman Emperor" on Christmas Day in St. Peter's Basilica. Charles (or "Charlemagne ") did control much of what had been the Roman western empire, but no provision was made for succession. In any event, Charles was quite content to just be king of the Franks. The pope crowned him as a way of thanking Charles for rescuing the papacy from the Lombards .
When Charles died, his "empire" was split among his sons, never to be united again. The title, however, lived on until 1806. As the old saying went, it was neither Holy (the pope was rarely consulted), nor Roman (most of the inhabitants were Germans), nor an Empire (except for a few decades under Charlemagne, it was an empty title without much power, not that many of his successors didn't keep trying to assert their power, occasionally with some success).
But because of the Roman legacy, and the church's continued use of Roman organizational techniques, much was passed down through the centuries. It was the Church, especially the monasteries, that preserved most of the literary works we now have from ancient times. Indeed, it was Charlemagne who, while encouraging literacy in general, commanded that monks in his lands start making copies of old manuscripts in their possession. Most of the ancient works that survive to the present were from these copies. Many of the legal and governing techniques used in Europe in the 14th century, and today, are direct descendents of what the Romans developed and used for centuries. While the church denounced the pagan beliefs of the ancient Romans, by the 12th century, the many Roman manuscripts available in monasteries were being avidly studied for practical and philosophical knowledge. And the Roman Catholic Church still uses Latin as a common language when clerics from many nations meet for conferences. Not the sort of Latin Julius Caesar would easily understand, but the ancient language of the Romans has survived because it was part of the administrative baggage the Christian church picked up when it became the official religion of the Roman empire.
Among the many items the early Christians picked up from the Romans was a love of learning and preserving knowledge. This was also a carryover from Christianity's Jewish roots. Indeed, it took a while before many Christians stopped considering themselves simply Jews who had accepted Christ as the messiah. The barbarian tribes lacked a written language (except for some crude runes) and the record keeping habits of the Romans. The barbarian leaders were quick to appreciate how useful all those Roman clerks and administrators could be. At this point, in the 5th and 6th centuries, all those Roman citizens were Christians and they served their new masters eagerly, while easing the way for mass conversions of the barbarians. Many of these Romans were Germans, or part German, their ancestors having entered the empire --peacefully or otherwise-- as part of earlier waves of German migration, and had been Romanized. The Romans were quick to give citizenship to anyone who would be a useful and agreeable subject. In this way, the Romans maintained their population over the centuries in the face of constant wars and living standards that weren't much better than those in 14th century Europe.
Once the barbarians were Christians, the barbarian nobles noted that they were very dependent on the local bishops and the trained clergy who provided the bulk of the educated managers. The feudal system, whereby the professional barbarian warriors defended the common people and the chergy in return for the right to rule and levy taxes, was developed with the encouragement of the church. It took several centuries for the hundreds of different barbarian tribes to sort themselves out and settle down. This was a messy process and the church preached, and desired, peace at just about any price. Feudalism was the result, but a side effect was that many senior clerics also became feudal lords. While Christianity preached love, many barbarian nobles joined the clergy and found it useful to raise an army when needed to restore peace. The church, as an institution, opposed this, but it wasn't until the 15th and 16th century that "fighting bishops" were completely done away with. By the 14th century the church had used its administrative skills to become enormously wealthy. Some 30 percent of the land in Europe was owned by the church, largely in the form of abbeys or lands held by feudal right by bishops, not to mention the Papal States, ruled directly by the Pope. Some bishoprics were so powerful that they were called Prince-Bishops, to indicate that they held civil as well as clerical power in their territory.
The wealth of the church attracted a lot of people who were more smitten by the money than by the faith. A major reason why the Church insisted on a celibate clergy from the 10th century on was to avoid inheritance disputes when married bishops or abbots died and their heirs claimed a portion of what was actually church property. The Church made the concept of clerical celibacy stick as a legal principle, but the practice was another matter. Many clergy kept mistresses. These women were supported with church funds their protectors controlled. The male offspring ("nephews") of prelates unions often found positions in the Church, the universities, or the law, while the female children ("nieces") might find themselves heading convents or married off to minor members of the nobility. Clerics also had brothers and parents who might want to share the church wealth, and often they did quiet nicely.
The constant struggle between the nobility and Rome over who would appoint bishops was largely a battle over who would control the church's wealth. It wasn't so much that the Curia Romana wanted to control all those rich bishoprics and abbeys, it couldn't. But the Church wanted to make sure that the clergy remained in the service of Christ, not the local nobility.
The Roman Catholic Church survived so long because it was decentralized. The Pope and the Curia gave general direction and maintained unity in the details of Christian belief. But the bishops ran their sees and the abbots their abbeys. The bishops and abbots would receive visitors from Rome on occasion, but the primitive communications of the time did not allow constant contact between the "head office" in Rome and all the outlying dioceses and abbeys. The local nobles realized that if they could get their clerical kinfolk made abbots and bishops, the local wealth of the church would be more likely to be available to themselves. While the church officially stood for church property being preserved for church use, the non-noble clerics were basically struggling with the nobility over control of these assets, of great importance inhelping to finance the work of the Church. While many of the clerics, no matter what their family background, were keen to do God's work, an increasing number had baser goals in mind. The noble clerics were more likely to see non-Church uses for all that clerical wealth. All of this came to a head when nobles began seizing church property outright (and becoming "Protestant" Christians in the process) in the 16th century. But in the 1300s and 1400s, the struggle was still unresolved. What was clear was the increasing use of the Church's enormous wealth to make life easier for the clergy, not the faithful served by the clergy and the Church. This became noticable after the Plague, an event thought to have killed a disproportionate number of the most religious minded clergy. There is a certain logic to this, as more lacadasical clergy would have fled the Plague, while their more devout brethren went to aid the afflicted.
Even before the plague, the abbeys and dioceses had been using a smaller and smaller portion of their wealth for charity and good works. While the church organizations were quick to collect a tithe (ten percent) of the produce of the laity under their control, they tended to spend far less than ten percent of their income each year on relief of suffering. While church teachings stressed charity, dioceases and abbey records from the period tell a different story. Most distributed only a few percent a year and after the Plague this tended to shrink even more. This increased clerical worldliness and other ecclesiastical abuses was repugnant to an increasing number of the clergy and the faithful. By the 15th century, vocal, and sometimes armed, dissent began to appear. By the 16th century, the heretofore monolithic Roman Catholic church cracked, and numerous new Christian churches split off from the Church of Rome. With that in mind, one can appreciate the Medieval period as a Golden Age for the Roman Catholic church. Power corrupts, and even the clergy can be so corrupted.