Part of medieval Christianity’s reworking of its inheritance from Classical Antiquity included turning the Greek Sibyls from local oracles into foretellers of Christ’s birth. After the christianized Sibyls’ prophecies had spent a thousand years or so on the medieval equivalent of the bestseller list, meddling philologists started asking just how the pre-Christian Sibyls came to know Jerome’s Vulgate so well. In the foreword to a German edition of sibylline and other assorted tracts printed in 1516 (VD16 ZV 11992), the anonymous author anticipates the question:
When you read the predictions and prophecies of these heathen women, you should not find it incredible that they proclaim our scripture about Christ so literally. For St. Augustine writes how he found in the heathen writings of Plato the great and profound gospel of John, from “In the beginning was the word and the word was with God” to “and the word became man.” Plato didn’t know anything to say about it. Also St. Thomas writes that in the histories of the Romans, in the time of Emperor Constantine and his mother Hierene a grave was found in Constantinople in which lay a man with a golden tablet on his chest upon which was written: “Christ will be born of a virgin, and I believe in him. O Sun, at the time of Hierene and Constantine you will see me again.” And when the grave was opened, the sun shone into it, and people saw the dead body lying there and the written prophecy about Christ the Lord, which was truly fulfilled in his conception and birth for our salvation.
The notion that people in the centuries B.C.E. knew of and believed in Christ, and wrote messages on golden plates for the benefit of people they knew would be living many centuries later, is, I gather, a bit embarrassing in polite society these days. One benefit of poking around old books is the occasional experience of recognizing a certain devotional kinship, of being able to say comrade, brother! to someone who, five centuries ago, also believed in embarrassing things.