For nearly four centuries before the fall of the Roman Empire, Christianity had become a popular and, when it was made the official Roman religion by Emperor Constantine in c. 313, influential religious movement. The Roman pagan deities had been in decline for centuries, giving way first to worship of Caesar, then to a relativist hedonism very similar to the worldview currently popular in the West. Most Roman citizens spent their days concerned only with the things of the world, the daily trials, duties and pleasures. They abandoned virtue - even as Christianity flourished, the common Roman had already lost all integrity, patriotism, patience and compassion. In fact, Christianity was ridiculed as a "woman's religion" due to the influential roles and high regard women held in the Church, a concept completely foreign to Roman paganism, and the charity of Christians visiting battlefields, tending to victims of both sides, was given a confused response of disgust and appreciation by Roman observers. The legions had forgotten their long-held national pride, military vigor and dignity, giving way to laziness and continual complaining to higher officers and on up to the Caesars who for the most part, before Constantine, were the epitome of the Roman decline.
Because of the heavy, if sporadic, persecutions Christians faced in the Roman Empire, their ability to evangelize outside Rome was very limited. After the Empire was split into the Western Roman and Eastern Byzantine Empires, and particularly after the conquest of Rome by the Goths, Christianity was opened up to the world. Despite the majesty of Byzantium, it reserved much of its splendor and power to the east, leaving the Roman Catholic Church as the torch of civilization, morality and truth in the West. Without borders, Catholic missionaries spread as far and wide as they could, risking (and often suffering) death in the quest to convert the Germanic, Gothic, Celtic, Britannic, Slavic and many other ethnicities of pagan peoples in Europe. At the advent of monasticism in the 500s, developing from Eastern desert hermitage and heralded by the Rule of St. Benedict, communities developed purely to prayer, holiness, charity service and especially to study spread across Europe, carrying Roman culture and Catholic religion to every part of the continent.
As can be seen by the example of Rome, European cultures have a general tendency as regards religious conversion, peculiar to itself: regardless of the popularity of a religion with the people, the whole conversion of a society depended heavily on the conversion of its leader. Christianity had been popular for centuries before Constantine, but only when he converted and made it the official Roman religion did it truly flourish. This was even more so true for other peoples in Europe over the centuries. Almost without exception, the religious allegiance of a society depended inextricably on the religion of their chieftain, king, emperor, etc. Catholic missionaries used many different methods to convert Europe, especially "sanctifying" the native religions by showing their people its similarity to and fulfillment in Catholicism, exemplified by St. Patrick, but the crowning act of European Catholic Christendom was the conversion of Charlemagne and his coronation on Christmas 800 by Pope Leo III. As King of the Franks and the first Holy Roman Emperor, he heralded a massive spread of art, culture, religiosity, morality and the advent of feudalism during what scholars call the Carolingian Renaissance. Under his Catholic leadership, Europe quickly became Christendom, with citizens, who had been leaning towards Catholicism for some time or had been leaning between it and their native paganism, following their leaders who took Charlemagne's cue.
As the religion of the kings and nobility, the Church was voluntarily given important and powerful roles in national politics during the Middle Ages. Cardinals and bishops often acted as chancellors, royal advisors and many other positions, while monasteries were the center of medieval society, representing their hospitals, hostels, colleges, documenters, historians and preservers of language, literacy and academia, particularly the remnants of antiquity. Many popes began in monasteries, and anyone seriously interested in academic studies became a monk. Nuns were equally important, being the primary physicians and caretakers of society. Over time, Catholic clergy and monastic officials with important roles in government and society received heavy financial endorsement of many kinds, including both real coin and goods such as land, livestock or other products, in the form of regular religious tithes and extraneous tokens of appreciation. These gifts usually had one of two motivations: genuine religious obedience and gratitude, or for political position due to these officials' importance in society and government. Often, tithing a cardinal with close ties to the king meant receiving favor and benefits from the king in return. As always happens, as Church officials became wealthy and powerful, being treated as almost supernatural themselves by the people, abuses crept up and spread corruption. This corruption has been largely overestimated over time, largely due to the propaganda of Protestants and others which, as propaganda always does, exaggerated to persuade and emphasize. But it did exist.
One of the main events which carried the Middle Ages into the Renaissance was the Black Plague. Primarily occurring in the 1300s and cropping up sporadically afterwards, it wiped out between 30-60% of the European population, reducing entire regions to vacancy. In the Middle Ages, life was difficult, turbulent and volatile, particularly due to war or other social causes such as poverty or corruption, but the people put their hope and faith in the Catholic Church and in its theology of God, believing that whatever hardships they faced, even death, Heaven awaited the obedient faithful. This worldview created a continent-wide devotion and deep religiosity that formed the center of people's lives. As the Black Plague began, this hope was punctured. For centuries the Church and the national monarchs had delivered the people from invaders, social problems and sin; but now, life seemed doomed to destruction with no end in sight, no cure, no salvation. Desperate, many began using pagan practices such as witchcraft and other superstitions, or the little medicine they knew, trying anything in their power. Many Catholics died in charity to help those inflicted with the Plague, but this did little to relieve the doubt on people's minds.
In the Middle Ages, the minds of people were always turned towards Heaven, even as the sweat of daily toil and pain weighed them down continually. Weekly, or even daily, Mass was the chance to get a preview of Heaven, with the wonderful art, music, bells, smoke, Bible readings, inspiring homilies and, finally, the jewel of the Eucharist, the true Life of Christ before and subsequently within them. But the Plague tore away people's focus from the Heavenly to the earthly, from the spiritual things of God to the physical things of man and nature. The unrelenting environment of death and horrible pain made people doubt and largely reject the hope they had once placed in the Church and her God. In desperation, they began to believe that the only viable, practical and truly moral focus was man and his world, to use it for his benefit - that only we could save ourselves, through power over nature.
This new worldview led into the Renaissance, expressed in its religion, philosophy and culture. Replacing the medieval art which depicted saints, angels and Heavenly things with man and nature; replacing the salvation of Christ through His Church with the industrious power of science; stealing goodness and holiness from God and giving it to a vague concept of "humanity" bound within us by political and religious tyrannies; and replacing egalitarian feudal economy with the mercantile which would lead to capitalism, the Renaissance had begun, and so the modern world.