score:20
I suppose you are talking about Plato's books. No major classical work from ancient Greece survived in the original. (Exception is some recently found papyri originating from the Roman Egypt, which are usually just small fragments). All those books that survived (and specialists estimate that about 1% of the ancient Greek and Roman literature survived) were copied, and we have much later copies or translations to other languages, mainly Arabic. The reason that you mention explains why so few survived. In most cases the earliest existing copies originate in 11-13 centuries or even later. Many books are available in several old copies, which of course differ, and a research was required to establish a standard modern text. The oldest existing copy of Plato's republic is made in 9s century.
Greek writers were not considered atheists (this word and the notion are 18s century inventions), they were considered pagans. Why some pagan authors were considered valuable enough to copy them? Because of the interesting information contained in these books. Christians understood very well that the authors who worked before Christ could not be Christians, so it is not their fault that they were pagans. But their writing could contain valuable thoughts and information, on the opinion of the Christians. Plato and Aristoteles, in particular were highly estimated by many Christian philosophers.
Very few ancient books were copied in Western Europe during the early Middle age. Most surviving copies came from the Eastern Roman empire, or through Arabic or Hebrew translations. But several authors, like Plato, Aristotle or Ptolemy were generally well-regarded in Europe, and their books were continuously copied in Western Europe.
Upvote:13
In Roman-Greek times (before 300 AD) there were no books in the Roman Empire, just papyrus scrolls, each chapter being one scroll. So a work like the Republic would be divided into many scrolls. Papyrus is relatively perishable, so virtually none of these scrolls survived, with only a few isolated exceptions.
The works have survived, however, because they were copied either by individual scholars or by being copied en masse in scriptoriums. In a scriptorium a master reader, called a lectorius, would recite the word and scribes would write it down as he spoke it. Another method of copying, more precise, was to use the punct, a sharp stick. With the left hand the copyist would place the point of the punct below the letter being copied and write it with the right hand. Then he would move the punct to the next letter and repeat the process. Our word "punctilious" comes from this process.
They began using parchment, the corium of a sheep, for the writing material, which is much more durable than papyrus. Note that the Romans did make some use of parchment, but the use expanded greatly in medieval times. Parchment was much, much more expensive than papyrus.
Every library in medieval times had a scriptorium. They would borrow scrolls or books, copy them for their library, then send them back.
At the Abbey of Cluny over 1000 ancient works survived by this method including those by Livy, Aristotle, Aesop, Horace and many others.