Angellous, what prompted my response was your question about "duty". It was their duty to preserve the historic record, and it is more than a little ridiculous that they would not preserve Celsus, given the effort they made to preserve the Christian critique of Celsus. You didn't ask me whether I expected them to have acted differently, and you were the one who raised this issue.
Censorship and alteration of historical records was not unique to Christians in those times. On balance, I think that the pagan-dominated Empire was more tolerant of diverse opinion than the Christian-dominated Empire. Christians were persecuted at times, but there were also periods of time when they were a tolerated minority. Libraries preserved works of diverse religious opinion. Under Christian emperors, Jews experienced less tolerance and book burnings of "blasphemous" works became publicly encouraged. Although the Library of Alexandria had suffered destruction during attacks before Christians came to power, it was systematically gutted for religious reasons by the Patriarch Theophilus in 391 AD. Previous acts of destruction had not been carried out specifically to purge it of content that was considered blasphemous by the authorities. That was in a time when the Christian Emperor Theodosius made paganism illegal in the empire and explicitly sanctioned the burning of pagan works.