Christianity in the Dark Ages was the Catholic Church. There was no other. Whether the monasteries operated independently of Rome does not mean that they were not Catholic. If you were a Christian, you were Catholic. The Church was one until 1054 when the Orthodox split.
See also Pyro7480’s post 73 which will remind you that rights and freedoms began in Catholic England with the Magna Carta in 1215.
This Catholic uptightness with other kinds of Christians is something that shines through the "Catholic radio" station in my home town. Rarely do the hosts or guests talk about converting non-Christians to christianity. Instead, they're focused on converting protestants to catholicism. I get the impression they're really bugged by the fact there are christians who aren't catholic - - but they're not that concerned about the "heathen" who don't believe in Christ to begin with. In contrast, protestant evangelicals put a lot more emphasis on reaching out to non-christians. Ironically, a lot of the converts to catholicism who are highlighted on this radio program, were first converted to christianity by protestant evangelicals. So the evangelicals seem to be a necessary "cog" in the Christian machine - but the Catholics on this radio station can't stop talking about how they're wrong and they must convert. It's that (soft) intolerance with other forms of Christianity that reminds me how things can be uncomfortable for Protestants in countries where Catholics are in charge.