"The valuable library of Alexandria was pillaged or destroyed; and near twenty years afterwards, the appearance of the empty shelves excited the regret and indignation of every spectator whose mind was not totally darkened by religious prejudice. The compositions of ancient genius, so many of which have irretrievably perished, might surely have been excepted from the wreck of idolatry, for the amusement and instruction of succeeding ages"
That's Edward Gibbon, in chapter 28 of his great work The Decline and Fall of the Roman Empire, describing how a Christian mob wiped out a great Classical library in the late 4th century AD when Christianity became the official religion of Rome. And Decline and Fall really is a great work. But Gibbon got this one wrong.
Yes, there was a very large library and school in ancient Alexandria, and at some ancient time, the library and school were no longer there. But when and how did they vanish?
Most likely, it was a gradual process of decline caused by neglect and lack of funding, punctuated by acts of destruction in wars. The greatest destruction of the library, after which there was hardly any library left to be destroyed, may well have occurred before there were any Christians, in 47 BC, when Julius Casar
invaded Egypt to help Cleopatra fight her brother, King Ptolemy XIII. There was a great fire in the docks of Alexandria when Caesar invaded, a fire which may or may not have burned down the library. Ancient accounts vary over whether the library was damaged or destroyed during Caesar's invasion. But one thing to keep in mind is the great number of references to the institution in the time before Caesar, from the 3rd to the 1st century BC, when it was the home of a thriving center of literary criticism and the collection, copying and correction of manuscripts, and the lack of such mentions afterward. This quite striking fact may mean that Caesar destroyed the library, perhaps intentionally, perhaps by accident -- or it may simply mean that even before Caesar arrived, there was not much of a library left to be destroyed.
Naturally, there were still books in Alexandria, and various public and private collections of books which would be described as libraries. But those may not have been the same as the great library which had been there before. References to the great Alexandrine library from this point onward can be interpreted as being expressed in the past tense.
In the third century AD three Roman Emperors attacked and sacked Alexandria: Caracella in 215, Aurelian in 272 and Diocletian in 295. The Christian Emperors beginning with Constantine in 313 did some damage to pagan literature, but mostly by neglect and lack of funding. In 391 Theodosius I made Christianity the official religion of the Empire and ordered all pagan temples to be closed. This certainly did the study of Classical literature no good at all, but there are simply no ancient accounts of Christians deliberately destroying Classical books.
Not even in Alexandria, where the pagan Sarapeum was destroyed some time after Theodosius' order. This destruction is the one which is popularly depicted as "the destruction of the great library at Alexandria" -- but in the five contemporary or near-contemporary accounts of the destruction of the Serapeum, by Rufinius Tyrannius, Socrates Scholasticus, Sozomen, Theodoret and Eunapius of Antioch, none of whom held back when describing other Christian atrocities, there is no mention of any books being destroyed along with the temple, let alone the greatest library the ancient world had ever seen.
But let us say, for the sake of argument, despite all of the above, that the greatest library the ancient world had ever seen really was in the Serapeum at Alexandria in AD 391, when it really was destroyed by a Christian mob which hated books and knowledge. The destruction of one book is an awful thing, even if it's not a very good book, and the destruction of thousands of books at once is a great horror. But even if such a horror occurred in Alexandria in 391 -- There were many other libraries in the ancient world.
Every major city had one or several collections of books open to the public. Private families often outdid the cities in collecting books and making them available to poets and scholars. The destruction of any one single collection, no matter how large, would not have equaled the wholesale destruction of Classical culture.
Yes, a great portion of ancient Classical literature is missing today, and yes, that is a very terrible thing, and, yes, Christian authorities have had something to do with that disappearance, although mostly by means of neglect, of lack of funding for the preservation and copying of Classical texts. But the rest of that story is that not all Christians leaders have been the least bit unfriendly to Classical studies. Many of them have been great supporters. Big sweeping statements one way or another just don't hold water on this topic.
As for what happened to that great library at Alexandria, which in the last few centuries BC was the greatest center of Hellenistic literary criticism, and which may or may not have been the largest collection of Greek scrolls in the ancient world: it, like Classical literature in general, probably died mostly by long periods of neglect punctuated by accidental disasters which were the by-products of wars. If there were any known instances of Christian mobs gleefully burning great big piles of scrolls containing Classical literature, I'd be the first to want to show you all of the evidence. I think Christianity has done a lot of really horrible things. Justinian closed the Platonic Academy in Athens in AD 529, a horrible thing to do. But this Christian destruction of Alexandria's great library -- sorry, my fellow atheists: we dreamed that one up.
And while I'm here being so fair to everyone, let's be fair to Gibbon, too. Yes, he got this one wrong. But that shouldn't take too much away from our admiration for the great number of things he got right.