Almost all of the works written in Latin which are the subject of Classical Studies were written by non-Christians. As far as I know, all but one were written in the very early 5th century AD or earlier. And then there is the Etymologiae, written more than two centuries later by Saint Isidore, Archbishop of Seville, who is usually depicted in paintings and other artwork holding a book.
Isidore was born ca AD 560, became Archbishop of Seville around 600, died in 636, was made a Catholic saint in 1598. He wrote many things which today are read mostly by theologians and historians of 6th-and 7th-century Spain -- and then there is his best-known work, the Etymologiae, an encyclopaedia in 20 books. Isidore worked on the Etymologiae for decades, was still working on it at the end of his life, and entrusted it to his friend Bishop Braulio of Saragossa, to finish it after his death.
Book 1 has to do with grammar, Book 2 rhetoric and dialectic, Book 3 mathematics, music and astronomy, Book 4 medicine, Book 5 law, Book 6 Christian books and Church offices, Book 7 God, angels and saints, Book 8 various Christian sects -- or, from Isidore's point of view, heresies, Book 9 languages and nations, Book 10 vocabulary, Book 11 the human body, Book 12 animals, Book 13 the cosmos, Book 14 the Earth, Book 15 buildings and fields, Book 16 stones and metals, Book 17 life in rural areas, Book 18 war and sports, Book 19 ships, buildings and tools, and Book 20 with miscellaneous supplies and implements.
It is called the Etymologiae, the Etymologies, after the 10th of these 20 books, and -- as in the cases of earlier authors like Valerius Maximuns, Pliny the Elder, Aulus Gellius, Quintilian, Macrobius and Servius, some of them also greatly prized as authors in their own right, others of them less so -- it is of great interest to Classical scholars because it quotes many pre-Christian authors, and in many cases it preserves passages from these authors which are otherwise lost.
Isidore is a great example of how there are exceptions to rules, and how things aren't always as simple as they seem. Almost all of the Classical Latin literature known to us today was copied at some point by Medieval Christian monks. However, by and large, the Dark Ages -- the earlier part of the Middle Ages, the time between the fall of the Western Roman Empire in AD 476 and the coronation as Emperor of Charlemagne in 800 -- were indeed quite dark to anyone interested in the preservation of ancient Latin literature. A very great amount of what we estimate to have been the finest of that literature disappeared during the Dark Ages, and to a great extent, it disappeared because a greater emphasis was given to the preservation of Christian literature. And yet, a significant amount of it has survived because of Isidore, a Christian archbishop living right in the middle of the Dark Ages, a contemporary of Pope Gregory the Great. Isidore, who was by no means a Christian in name only, taking advantage of a cushy Church position in order to pursue Classical Studies. Isidore, who took a very active part in shaping the Christian theology and politics of his time.
It's true that a great deal of what Isidore compiled is taken from earlier compilers. Isidore takes quite a lot, for example, from Servius. (Servius (late 4th century -- early 5th century) called his work, which is thousands of pages long, a commentary on Vergil; others have opined that it is in fact an encyclopaedia which happens to be arranged in the order of passages from Vergil). But it's also true that Isidore took much of his material from manuscripts of individual Classical authors -- for instance, if we are to believe David Butterfield, The Early Textual History of Lucretius' De Rerum Natura, Cambridge: 2013, pp 89-91, and I think we ought to, Isidore had a complete manuscript of Lucretius' Epicurean poem, a work which some have alleged was shunned by the entire Christian world between Antiquity and the Renaissance. If the many passages from Lucretius in the Etymologiae don't already make Isidore's high esteem for Lucretius clear, he has given us another big hint in the title of one his other works: De natura rerum. A Dark Age archbishop and future saint made it plain, in a work which was extremely popular throughout the Middle Ages, a work of which more than a thousand Medieval manuscripts survive (a thousand is a lot), that he held Lucretius in very high esteem.
Things are definitely not always so simple and clear-cut as some would have you believe.