How did the Catholic Church grow in power between Rome and the Middle Ages?

Question

I am not a historian, but I love learning history, and have loved it since middle school. One thing that was odd to me even back then, though, is that there seemed to be a "gap" in European history.

What I mean by this is that we studied quite a lot about Rome, from its early monarchy, to the republic, to the empire and the later fall, around the fifth century. During the empire, one noteworthy occurrence was the gradual rise of Christianity, until it became the official religion of the empire in the fourth century. This is already a process I would really love to learn more about, but my focus is on what happened next. We would briefly look at the peoples that formed the Germanic kingdoms in the former Roman lands, and then would go straight to the eighth century and Charlemagne, which would then go to the Middle Ages.

It seemed (and still seems) to me like in this "gap", between the fall of Rome and the rise of the Carolingian empire, in between the fifth and eighth centuries AD, the Catholic Church somehow went from "a religion growing among the Roman populace and adopted as the religion of the empire" to "the dominant force in pretty much all geopolitical matters". Somehow, the Church acquired tremendous influence, power and wealth in this period, which is what eventually led to the complete religious dominance for centuries during the Middle Ages.

What I would like is for some references to read on how the Church grew so much in power, and what exactly happened between the fall of Rome and the beginning of the Middle Ages to reshape society in such a fundamental level.

Answer 1

Bret Deveraux's Decline of Rome, especially part 2, is well-sourced and answers this question pretty well: the Church did not grow in power per-se, but rather was Rome's most enduring institution as the others degraded:

...the institutional Church was in some ways a lifeboat in which other elements of the Late Roman world were carried through the storm of the fifth century into the Middle Ages.

As barbarian tribes moved into Rome, they displaced remote Imperial rule with local kingly rule - but continued to rely on local Roman elites to keep administration going smoothly.

some of the Gallo-Roman elite retreat into their books and estates, while more are co-opted into the administration of these new breakaway kingdoms, who after all need literate administrators beyond what the ‘barbarians’ can provide. Mathisen notes that in other cases, Gallo-Roman aristocrats with ambitions simply transferred those ambitions from the older imperial hierarchy to the newer ecclesiastical one

The Chalcedonian Christian Church was already extremely wealthy as the recipient of the Emperor's sponsorship, but now it rapidly gained temporal power as well. The king was a barbarian and ruled from his castle, but to the bishop went the governance of monasteries and cities:

While the figure of the bishop rose to importance in both the East and the West, in the East, bishops largely did not move into becoming secular leaders alongside religious figures, in part presumably because the central Roman authority still existed to handle those functions. But in the West, the decline of both civic and centralized government left bishops in the breach.

Of course, we should not underestimate the power of belief either. People believed their religion and so once the Church - not only the source of learning but also salvation - began to contest secular power, it was relatively successful.

However, as Deveraux notes, that only really began to take off in the 11th century so it's a bit beyond your 5th-8th century timeline. In the 8th century, the Vatican needed the Emperor's protection; in the 11th it dictated terms to the Emperor.