I'm going from what I learned in my history of western civ class more than a decade ago. But it seems like Wikipedia vaguely suggest that this is one possibility (
https://en.wikipedia.org/wiki/Library_of_Alexandria). It's the first I've heard any differently, honestly. Do you have a source?
Paganism was made illegal by an edict of the Emperor Theodosius I in AD 391. The temples of Alexandria were closed by Patriarch Theophilus of Alexandria in AD 391.[32] The historian Socrates of Constantinople describes that all pagan temples in Alexandria were destroyed, including the Serapeum.[34] Since the Serapeum housed a part of the Great Library, some scholars believe that the remains of the Library of Alexandria were destroyed at this time.[32][35] However, it is not known how many, if any, books were contained in it at the time of destruction, and contemporary scholars do not mention the library directly.[36][37] |
In any case, that wasn't the main point. The main point is that Muslims preserved some of the ancient classics, which deserves to be figured into any history of Islam.
...and college professors...
It was a barbaric past, which included religious wars between Christians and persecutions of religious minorities as well as feudal rule. Part of that is simply down to the passage of time and the inevitable shifting of standards, but some of it is really, fairly objectively worse than today if you care about free open societies or human rights as currently understood.