Just earlier today I wrote loong post in this thread on a somewhat related topic, and it's late here so I'll just add some short comments (and if anyone else wants to elaborate further, be my guest)

Basically you're having trouble here reconciling the respect shown in the Middle Ages for Rome, with the notion they were "hyper-fundamentalist". The answer is that your conception of their religiosity is wrong. Now the Catholic Church was strong in the Middle Ages, people were more devout, religion played a larger part in peoples lives than it did today. Religious laws against things like fornication and adultery were often brutally enforced, and stepping out of line in specific ways that may seem innocuous to a modern reader could get you into deep trouble.

But the church and clergy were not 'fundamentalists' in the way that some modern-day denominations are. Today such groups ban or protest Harry Potter books because they describe witchcraft. A medieval denizen would normally agree that witchcraft is bad and should be punished. But they would also agree that things described in those books, such as the Philosopher's Stone and the poison-curing powers of bezoars, were real things that existed and not witchcraft. Witchcraft was something many believed they could protect themselves against using magic, i.e. protective amulets, talismans and prophylacteries. That might sound like a contradiction to modern reader, and it was indeed complicated, but even the later Puritans in North America during the renaissance were involved with plenty of folk magic even as they prosecuted witches. (see e.g. Richard Godbeer, "Folk Magic in North America") So it was not just restricted to the Medieval era. Nor was it restricted to the Christians. Among the Jews you had Kabbalah, and even a common practices like the mezuzah is (from a historians perspective) an amulet, and even the Star of David has its origins in Jewish magical texts. It was only adopted as a Jewish symbol fairly recently, in the 19th century (for more on all this, see for instance Trachtenberg - Jewish magic and superstition _ a study in folk religion, 2013) The Islamic world, too, had magical practices, and there was really a quite surprising amount of interchange of ideas between these communities that were otherwise very segregated and bigoted toward each other. But I digress.

Point is, there was quite a bit of heterodoxy during the Middle Ages. Lots of people did lots of stuff that many Christians today - or in that day - would not consider proper. Yet it was not for lack of religious devotion they did that stuff.

As I said in the other post, there were similarly varied opinions on how to relate to the pagan past. As a concrete example, I took the pagan week-day names, where Bishop Ögmundsson of Iceland successfully renamed them (but was alone to do so), while for instance Saint Isidore of Seville seemed to feel the pagan names could be tolerated even if inferior to Christian names.

Perhaps you are underestimating the religious intolerance here. These pagan faiths were not some competing religious system that had to be respected. They were a delusion, an error. Demons having tricked men, over generations to worship other powerful men of the past as gods. (This euhemerism was common in the Middle Ages) Anything good the Romans and Greeks achieved was in spite of being pagans, not because of it.