This subject is way out of my expertise, and I am having trouble verifying any of it. I am not even sure asking why the Roman Empire fell is a good question. A better question might be why it lasted so long. It did not really fall around 400 AD; it moved east and became the Byzantine Empire. It was weakened by Islamic invasions more than anything else.
The Dark Age was mainly dark in the sense of a lack of historical written records. Some Roman technology was lost in some areas, but historians argue that intellectual and technological progress continued.
If Christianity were somehow the cause of a collapse, then I would expect the collapse to be greatest in the more Christian areas. But history says the opposite. The collapse was greatest in areas like Britain, which were not Christian at all in the Dark Age.
So I do not understand why Christianity is blamed. Christians were not burning books, or forbidding scientific experiments, or anything like that. Sometimes it is claimed that Christians disallowed autopsies and dissections, but that seems completely false.
Starting in around 630 AD, the Mohammedans waged war against Christians in the Byzantine/Roman empire, and soon in Romanized western Europe. They also invaded the Persian empire, and became the world's biggest empire.
The Muslim Times argues:
The blame of Dark Ages should be placed squarely on the irrationality and coercion preached by the ChurchThe source is the Encyclopedia Britannica article on the Middle Ages:
The sack of Rome by Alaric the Visigoth in 410 ce had enormous impact on the political structure and social climate of the Western world, for the Roman Empire had provided the basis of social cohesion for most of Europe. Although the Germanic tribes that forcibly migrated into southern and western Europe in the 5th century were ultimately converted to Christianity, they retained many of their customs and ways of life; the changes in forms of social organization they introduced rendered centralized government and cultural unity impossible. Many of the improvements in the quality of life introduced during the Roman Empire, such as a relatively efficient agriculture, extensive road networks, water-supply systems, and shipping routes, decayed substantially, as did artistic and scholarly endeavours. This decline persisted throughout the period of time sometimes called the Dark Ages (also called Late Antiquity or the Early Middle Ages), from the fall of Rome to about the year 1000, with a brief hiatus during the flowering of the Carolingian court established by Charlemagne. Apart from that interlude, no large kingdom or other political structure arose in Europe to provide stability. The only force capable of providing a basis for social unity was the Roman Catholic Church. The Middle Ages therefore present the confusing and often contradictory picture of a society attempting to structure itself politically on a spiritual basis. This attempt came to a definitive end with the rise of artistic, commercial, and other activities anchored firmly in the secular world in the period just preceding the Renaissance.I guess the Muslim Times does not agree with this separation of church and state powers, but this was essential to modern civilization.
After the dissolution of the Roman Empire, the idea arose of Europe as one large church-state, called Christendom. Christendom was thought to consist of two distinct groups of functionaries: the sacerdotium, or ecclesiastical hierarchy, and the imperium, or secular leaders. In theory, these two groups complemented each other, attending to people’s spiritual and temporal needs, respectively. Supreme authority was wielded by the pope in the first of these areas and by the emperor in the second. In practice, the two institutions were constantly sparring, disagreeing, or openly warring with each other. The emperors often tried to regulate church activities by claiming the right to appoint church officials and to intervene in doctrinal matters. The church, in turn, not only owned cities and armies but often attempted to regulate affairs of state.
Edward Gibbon supposedly wrote the last word on the fall of the Roman Empire, but he was a big Christianity blamer.
Gibbon's work has been criticised for its scathing view of Christianity as laid down in chapters XV and XVI, ... More specifically, the chapters excoriated the church for "supplanting in an unnecessarily destructive way the great culture that preceded it" and for "the outrage of [practicing] religious intolerance and warfare".It makes no sense to me to blame Christianity for a fall of a civilization, when even Christian society is more civilized than every non-Christian society. Christian culture turned out to be much greater than the Roman culture preceding it, by any measure. If anything, Christendom was too tolerant of Islam, as Islam was an existential threat.
Some people have their gripes about Christendom, and some claim that it had fallen behind the Islamic empire, China, and India during the Dark Age, but it created modern civilization. Christianity promoted individualism, pluralism, rule of law, free will, pursuit of truth, science, and many other essentials. These things took centuries to develop, and they did not all develop elsewhere without Christian influence.
It seems possible to me that the only religious beliefs holding back Europe during the Dark Age were that of Islam. Because of the Islamic empire, Christendom was frequently defending itself against Mohammedan invaders, and losing territory and trade routes.
Maybe someone could have said that Islam was the superior religion in 900 AD, as it was gaining power and territory and Christendom was losing. That would be about like saying in the 1930s that Communism was superior to Capitalism. Maybe it looked that way to a superficial observer. But nobody could say anything so ridiculous now.
Now Moslems are threatening to overrun Europe again. Europe seems to lack the will to defend itself. I wonder if they even realize how Christianity helped get them where they are today.