After the Athenian Alliance in Greece lost its power, Rome (in Italy) became the dominant and imperial civilization in Europe and the Mediterranean. Rome and, therefore, Europe became Christian when Constantine, the Roman emperor, converted to Christianity. Conversion for ordinary Europeans was forced on them; eventually they embraced Christianity but initially they were reluctant to give up their old pagan religion which was a worship of a fertility god. The early Christian church was sensitive to this and very clever. The Church made it easy for people to switch to the new religion by combining the pagan celebrations with the Christian: to this day, we still have eggs and bunnys (symbols of fertility) associated with the resurrection of Christ; Christmas is celebrated Dec. 25 because around this time was the celebration of the winter solstice, also devoted to the celebration of agrarian fertility.
Rome was a successful administrative civilization and brought law, government, roads, irrigation, etc. to the primitive countries of Europe; however, they, like the great civilizations before them, eventually collapsed. Why? War, of course. This time, war brought on by the barbarian Germanic tribes of Germany and Scandinavia (people like the pagan Beowulf) that swept through Europe wiping out all that the Romans had accomplished. And so humankind begins another Dark Age, much like the Mycenaean Greeks' after the collapse of their civilization in the twelth century B.C.E. The "Dark Age" in Europe lasted until what we call the Middle Ages (or the medieval age), from roughly 500-1500 A.D.
From an historical perspective, the new religion appealed to
Psychological factor: Christianity -- with its messages of forgive thy neighbor; turn the other cheek -- offered a release from hatred and an answer to the problem of revenge.
During the Dark Ages, the Church became an organizing bureaucracy and provided stability, internationalization and a meritocracy. Also, learning -- including literature -- survived in Christian monasteries where monks and labored in scriptoriums copying ancient texts that would otherwise be lost to us. They also "illuminated" the pages of these texts.