"Christians precipitated the destruction of the pagan culture, which was the only culture shared by all people in Europe."
? 'Pagan culture' is not some unified set of cultural norms. Rather, it's a broad category of possibly totally unrelated activities. Some of them codified via Rome (but those were adapted to Christianity) and then other, local ones.
The Christians in 300 CE were the most organized group on the continent, which was part of the reason the Emperor adopted Christianity.
An organized group of busy bodies might cause the decline of some thing (i.e. paganism) but certainly not the decline of the written word, education, governance etc. just the opposite.
One might argue that with the failure of Imperial Order, and the onset of tribal wars, it was the monks that carried most of the flame of civilization, which eventually led to the renewal of civil order and the reasonable ability to establish something approach civility.
Which is maybe close to the classical 'dark ages' narrative. Now of course maybe it was not as dark as we thought, but we certainly don't have a lot leftover from that time. Hence the historical narrative.
