The term sibyl refers to the prophetesses that lived in history. They were pagan, but as Christians are wont to do, they Christianize pagans. The word itself is Greek for prophetess. They were strong women who spoke out. They were named after places they lived, i.e., Delphic Sibyl, Persian Sibyl, etc.
Some of these Sibyls are mentioned in Christian history. "You may see some on altarpieces and illuminated manuscripts and even on the ceiling of the Sistine chapel, the periphery of which is dominated by five Sybils (the Delphic, Cumaean, Libyan, Persian, and Erythraean) interspersed with seven Old Testament Prophets (Zacharias, Isaias, Daniel, Jonas, Jeremias, Ezechiel, and Joel).
Pay attention to the words in 'Dies Irae,' sung at Masses for the dead. It's opening lines:
|Dies irae, dies illa,|
solvet saeculum in favilla,
teste David cum Sibylla.
|That day of wrath, that dreadful day,|
shall heaven and earth in ashes lay,
as David and the Sybil say." *
Does it bother you that we Catholic Christians seem to honor these pagan myths? It shouldn't. As I said earlier, Christians baptize pagans. St. Thomas Aquinas teaches us that there's truth in everything. Look for that. These Sybils remind us that the Church teaches that actual grace and the natural virtues exist outside of the Church and we Catholics are to honor Truth no matter where it comes from. Did not Aquinas learn from Socrates? Arrogance and spiritual pride have no place in a Catholic. We shouldn't presume to know whom God blesses. The Holy Spirit may bless anyone by sanctifying grace. The best and most wise way to follow would be to treat everyone equally with charity. Honor Truth wherever you find and live your life according to God's commandments.
* Thanks for the information found on https://www.fisheaters.com/sybils.html