Societies usually change due to external forces incapable of assmiliation without creation of a new paradigm and frankly, that's what happened here.
I would submit that the Roman political ideal was crumbling before Christianity appeared and that it's eventual utter breakdown half a millennium later had more to do with it's exposure to a far more 'virile' and utterly different agent in that of the Germanic tribes who invaded and took her over than with it's eventual Christianization. Indeed, it can be argued that it was Christianity which represented Romanitas at the time of the invasions and which eventually succeeded in assimilating the barbarians, effectively Romanizing them.
This is true of the west more specifically than the east which remained identifiably Roman for quite a bit longer.
And this supports my point. The eastern empire remained identifiably and comfortably Roman as well as Christian both well into the second millennium of the common era.
As for the "casualties of Chritianity's expansion". Have you not read where Pompey raided the temple in Jerusalem, taking all the plate and melting it down to pay his legions? Or, of Caesar's brutal suppression of the druidic culture in Gaul? Or Lucullus' razing of Tigranocertes and the virtual extinction of Armenian culture there?
The utter destruction of weaker cultures, with their physical expressions by stronger ones is an historical commonplace, without reference to creed.
From this I would conclude that while marks of a culture are often lost in processes like these, it cannot be denied; the ideals the things represented are not always lost. In the case of the classical ideals you mentioned though their marks were lost, the ideals these marks were the expression of sprang again into full flower in the renaissance, eventually finding their place within (rather than opposed to) a distinctly Christian worldview.
The struggle was enormous, but it was successful.
I hope you can understand this. I am not an academic and suffer from muddle-headedness and poor self-expression as from a disease.