Your question is unclear to me. The western Roman Empire fell in 476. The eastern Roman Empire fell in 1453. Could you clarify, please?

Also, if you could explain what you mean by

a period in which people fought for their land/region rather than adopt new religions

it would also be helpful, as I do not see these things as mutually exclusive.

In Late Antiquity with the fall of the Western Roman Empire, the Church in the West largely substituted the aristocratic career. Bishops rose at this time as curiales, meaning that they could:

• Organize military defenses
• Negotiate with the enemy
• Ransom prisoners through the sale of church property
• Repair failing infrastructure
• Arrange for shipments of food in a time of rampant poverty

All of which was during a time of great confusion in the wake of the dissolution of major cities and an advanced political structure yielding to Germanic kingdoms. There was an edict ca 396 -- before the Sack of Rome in 410 -- that stated, "We (Emperors Honorius and Arcadius) command that all decurions shall be warned under threat of punishment that they must not flee or desert the municipalities for the purpose of dwelling in the country. They shall know that if they prefer any farm to a municipality, such farm shall be confiscated to the treasury, and they shall be deprived of that country district for which they prove themselves impious by avoiding their own municipality" (CT 12.18.2), which shows that even before the Sack of Rome, the political structure was falling apart. In this absence, the Church stepped in -- a natural progression from the rise of the bishopric during the 4th century.

Also the rise of the Benedictines' monastic monopoly did more than acquire land, it also standardised monasticism during the Carolingian Renaissance, which increased the Church's role as educators of the youth and as a source of leadership, as well as a center of wealth, all of which would have greatly increased the Church's prestige and influence.