History might credit him with a much darker deed though, not surprisingly one the church usually fails to mention or attempts to brush under the rug. Although not official of course, some have referred to him as the patron saint of arsonists. His leadership possibly brought about what is probably the single most destructive annihilation of knowledge in human history.
The Great Library of Alexandria is thought to have been founded around the beginning of the third century BCE. A museum, a research institute, and a library, it was the first of its kind. Besides attempting to collect all the world's knowledge, it housed a number of esteemed scholars; and boasted the latest works in mathematics, astronomy, physics, natural sciences and other subjects. It was at the Library of Alexandria that the scientific method was first conceptualized and utilized.
There are several descriptions of the destruction of the Library of Alexandria, all from different periods in time. First in 48BCE by Julius Caesar, then by the attack of Aurelian in the third century CE, by the decree of Theopilus (Cyril's uncle) in 391CE, by the mob incited by Cyril's followers around 416CE, and finally by the Muslim conquests around 642CE. Its probable that all of these caused damage to the library in some form or another; but historians have been as-yet unable to say when its complete destruction came about. When it was destroyed though, one of the greatest repositories of knowledge was lost to humankind; somewhere between 600,000 and 1 million volumes of knowledge were lost, many of them forever.
The hypothesis that Cyril's followers destroyed the library stems from stories of the murder of Hypatia. Hypatia of Alexandria was a Greek scholar who taught philosophy and astronomy at the library, as well as being a notable contributor in the field of mathematics. And a woman; something completely unheard of at the time. Hypatia came into conflict with Cyril, first because of her friendship to Orestes (the prefect of Alexandria, and stanch resistor of Cyril's agenda for a church-ruled government); and secondly because of the church's view of many sciences being heretical or associated with paganism. Hypatia was murdered in 415 at the hands of a Christian mob led by Cyril's assistant, after being incited by Cyril to oppose Orestes and his allies. Without her leadership to defend the library, it was only a matter of time before Cyril was able to convince and incite his followers that the libraries association with paganism and heresies deemed it an enemy of the church and a target for destruction.
Here is an excerpt from Cosmos where Carl Sagan talks about the Library of Alexandria:
If a story has conflict (especially religious or political), with a fight, a murder, and little intrigue... You know Hollywood is going to take a crack at ruining it. And this story is no exception of course. Agora has already been released in a few locations in Europe; but oddly has no US release date. The movie sets the time in 391CE, with the main character as Hypatia; but also including Theophilus, Cyril, Orestes, and a Muslim invasion. So I would speculate that Hollywood is blending three of the destruction stories.
A last little side note: Some historians speculate that the library may have contained actual writings of Jesus (instead of all the third-hand stories). A little conspiracy theory, I know; but if Jesus existed and he did write, how different might that writing be than what the Catholic church collected and distributed?
Information pulled from here, here, here, here, here, here, and here.