The genre of the Sibylline Oracle is well known in the ancient world. The Sibyl is always an elderly woman who delivers strange sayings as if from the gods. Ovid tells the story of a woman who asked Apollo to live as many years as there are sands on the seashore. The wish was granted, but she did ask the god to keep her from aging, so she is forced to live as a shriveled old hag. Various cultures have versions of this story – the Jewish legend calls her Sabbe or Sambethe and made her a daughter of Noah (Collins, OTP 1:317-38)
There were many sibyls by the fourth century B.C., but by the first century B.C. the most important was the Roman Sibyl. Her sayings were kept in Rome and consulted in times of crisis. These books were destroyed in 83 B.C. when the temple of Jupiter was burned. When it was rebuilt in 76 B.C., sibylline books from all over the empire were brought to Rome to be housed at the temple. Roman sibylline texts were filled with omens and prodigies, so too the Jewish oracles.
When something strange happened, the Oracles were scoured to give potential meaning to the event. The books could function as propaganda since a king could confirm his action by pointing to an arcane sibylline line which “predicted” his birth or some other key event. The obscurity of these works made them easy to manipulate and fabricate (Cicero, De divinatione 2.54.110; Plutarch, De pythiis oraculis, 25, cited by Collins 1:320, note 38). Eventually Augustus destroyed thousands of Roman oracles because he considered them politically subversive (Collins, OTP 1:320, citing Suetonius, Augustus 31.1.
The collection of oracles titled Sibylline Oracles in most collections of the pseudepigrapha are Jewish or Christian creations which mimic the style of Roman oracles in order to provide some additional validity to Jewish (or Christian) worldviews. The Sibylline Oracles are not single work from any one time. They range from Jewish works of the first century to late Christian theologies. To complicate matters, there are Christian interpolations into some of the Jewish oracles. This is a real problem for using this material: what is (early) Jewish as opposed to (later) Christian?
Sometimes this is obvious since the writer is clearly referring to Jesus Christ. For example, in the eighth oracle, lines 217-250 “an acrostic poem that spells out with the initials of each line the words Iēsous Christos Theou Huios Sōtēr Stauros, “Jesus Christ, Son of God, Savior, Cross.” As Collins points out, the first five of these letters spell Ichthus, “fish,” a famous Christian cryptogram (OTP 1:416).
Other times it is possible we may have a vague reference to a messianic figure or the messianic age which could be either Jewish or Christian. For example, in Oracle 3, some elements seem Jewish, such as lines 573-75, “There will again be a sacred race of pious men who attend to the counsels and intention of the Most High, who fully honor the temple of the great God.” But a few lines later there is a description of a restored kingdom which sounds like Christian descriptions of a millennium: “And then God will give great joy to men, for earth and trees and countless flocks of sheep will give to men the true fruit of wine, sweet honey and white milk and corn, which is best of all for mortals (3.-619-622).
Many times these “either/or” sections are not important (praise of God, for example), but in eschatological contexts it is very difficult to tell the Jewish from the Christian. This is the case because early Christian eschatology is very similar to Jewish eschatology since both developed out of the Hebrew Bible.