Why are there Sybils and Prophetesses in the Sistine Chapel?
For a short answer: They were the ancient pagan prophetesses of old who predicted the coming of Jesus into the world.
A Sibyl was an oracle or prophetess in ancient Greece who was known to prophesy at holy sites under the influence of a deity. The first collection of Sibyl utterances were recorded by Roman author Lactantius in the early 4th century AD. Sibyls were represented in art as early as the Middle Ages as well as early Renaissance pieces. Varro numbered ten Sibyls though other ancient sources differ as to the number, some only list one while others as many as twelve.
On the ceiling of the Sistine Chapel, Michelangelo alternated five Sibyls and seven prophets. The five Sibyls painted by Michelangelo were said to have foretold of the birth of a saviour. The prophecies by the pagan prophetesses were accepted by Christians as being fulfilled with the birth of Christ. Thus the prophets of the Old Testament and the Sibyls of pagan antiquity all foretold the same coming of the Christ and are depicted together in the Sistine Chapel. Michelangelo painted these figures larger than any other in the Sistine Chapel. During Michelangelo’s time there was a renewed interest in the writings of the classical and early Christian period which drew more attention to the Sibyls. Michelangelo’s depiction of them here shows the shift in theology at the time.
The Sibyls Michelangelo painted are not all beautiful but they all have a sense of being powerful. The paintings are like sculptures as their robes seem to billow around them. He painted the Cumaean, Delphic, Erythraean, Persian and Libyan Sibyls. Why he selected the five Sibyls he did is not known though a number of different theories do exist. - Michelangelo’s Sibyls of the Sistine Chapel
Let us not forget that even within the sacred liturgy of the Mass of the Extraordinary Form and in particular the now famous Mass of Requiem there appears a Latin sequence that is sung in Gregorian Chant and mention the Sibyls in the Dies
Dies Irae is a Latin hymn or sequence prescribed for the Roman Catholic Requiem Mass (Mass for the Dead or Funeral Mass) as well as on the Feast of All Souls (November 2) until the liturgical reforms which followed the Second Vatican Council. It could be noted that there are more translations in English than in any other modern language of this hymn. By 1913, the English renderings numbered 234, according to the Catholic Encyclopedia's article on the subject.
Fish Eaters has this to say about King David and the Sibyls:
These women are often depicted in medieval dramas, Jesse Trees and Nativity scenes. One hears of the Sibyls in Catholic chant and hymns, too: on Christmas Eve, after Matins and before Mass, The Song of the Sibyl was sung all over Europe until the Council of Trent (now this custom, restored in some places in the 17th c., remains mostly in Spain). They are most famously mentioned in the "Dies Irae," sung at Masses for the dead. Its 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.
Who were these women whom Christians group with King David and the great Old Covenant Prophets? Why did Tertullian (b. ca. A.D. 160) describe one Sibyl as "the true prophetess of Truth"? 2 Why would St. Clement of Alexandria (d. ca. A.D. 215) describe a Sibyl thus in Chapter VIII of his "Exhortation to the Heathens":
Let the Sibyl prophetess, then, be the first to sing to us the song of salvation:
"So He is all sure and unerring: Come, follow no longer darkness and gloom; See, the sun's sweet-glancing light shines gloriously. Know, and lay up wisdom in your hearts: There is one God, who sends rains, and winds, and earthquakes, Thunderbolts, famines, plagues, and dismal sorrows, And snows and ice. But why detail particulars? He reigns over heaven, He rules earth, He truly is."
- where, in remarkable accordance with inspiration she compares delusion to darkness, and the knowledge of God to the sun and light, and subjecting both to comparison, shows the choice we ought to make. For falsehood is not dissipated by the bare presentation of the truth, but by the practical improvement of the truth it is ejected and put to flight.
The Libyan Sibyl
The Delphic Sibyl
The Delphic Sibyl
The Erythraean Sibyl
The Persian Sibyl
No one truly knows why Michelangelo chose to include the portraits of these five (5) Sibyls in the Sistine Chapel! There seems to be no historical writings of Michelangelo on his reasoning for doing so!
Gallery of Sistine Chapel Ceiling