|Practices and beliefs|
The Sibylline Books (Latin: Libri Sibyllini) were a collection of oracular utterances, set out in Greek hexameters, that according to tradition were purchased from a sibyl by the last king of Rome, Tarquinius Superbus, and were consulted at momentous crises through the history of the Republic and the Empire. Only fragments have survived, the rest being lost or deliberately destroyed.
The Sibylline Books should not be confused with the so-called Sibylline Oracles, twelve books of prophecies thought to be of Judaeo-Christian origin.
According to the Roman tradition, the oldest collection of Sibylline books appears to have been made about the time of Solon and Cyrus at Gergis on Mount Ida in the Troad; it was attributed to the Hellespontine Sibyl and was preserved in the temple of Apollo at Gergis. From Gergis the collection passed to Erythrae, where it became famous as the oracles of the Erythraean Sibyl. It would appear to have been this very collection that found its way to Cumae (see the Cumaean Sibyl) and from Cumae to Rome.
The story of the acquisition of the Sibylline Books by Tarquinius is one of the famous legendary elements of Roman history. The Cumaean Sibyl offered to Tarquinius nine books of these prophecies; and as the king declined to purchase them, owing to the exorbitant price she demanded, she burned three and offered the remaining six to Tarquinius at the same stiff price, which he again refused, whereupon she burned three more and repeated her offer. Tarquinius then relented and purchased the last three at the full original price and had them preserved in a vault beneath the Capitoline temple of Jupiter. The story is alluded to in Varro's lost books quoted in Lactantius Institutiones Divinae (I: 6) and by Origen, and told by Aulus Gellius (Noctes Atticae 1, 19).
The Roman Senate kept tight control over the Sibylline Books; Sibylline Books were entrusted to the care of two patricians; after 367 BC ten custodians were appointed, five patricians and five plebeians, who were called the decemviri sacris faciundis; subsequently (probably in the time of Sulla) their number was increased to fifteen, the quindecimviri sacris faciundis. They were usually ex-consuls or ex-praetors. They held office for life, and were exempt from all other public duties. They had the responsibility of keeping the books in safety and secrecy. These officials, at the command of the Senate, consulted the Sibylline Books in order to discover not exact predictions of definite future events in the form of prophecy but the religious observances necessary to avert extraordinary calamities and to expiate ominous prodigies (comets and earthquakes, showers of stones, plague, and the like). It was only the rites of expiation prescribed by the Sibylline Books, according to the interpretation of the oracle that were communicated to the public, and not the oracles themselves, which left ample opportunity for abuses.
In particular, the keepers of the Sibylline Books had the superintendence of the worship of Apollo, of the "Great Mother" Cybele or Magna Mater, and of Ceres, which had been introduced upon recommendations as interpreted from the Sibylline Books. The Sibylline Books motivated the construction of eight temples in ancient Rome, aside from those cults that have been interpreted as mediated by the Sibylline Books simply by the Greek nature of the deity. Thus, one important effect of the Sibylline Books was their influence on applying Greek cult practice and Greek conceptions of deities to indigenous Roman religion, which was already indirectly influenced through Etruscan religion. As the Sibylline Books had been collected in Anatolia, in the neighborhood of Troy, they recognized the gods and goddesses and the rites observed there and helped introduce them into Roman state worship, a syncretic amalgamation of national deities with the corresponding deities of Greece, and a general modification of the Roman religion.
Since they were written in hexameter verse and in Greek, the college of curators was always assisted by two Greek interpreters. The books were kept in the Temple of Jupiter on the Capitol, and, when the temple burned in 83 BC, they were lost. The Roman Senate sent envoys in 76 BC to replace them with a collection of similar oracular sayings, in particular collected from Ilium, Erythrae, Samos, Sicily, and Africa. This new Sibylline collection was deposited in the restored temple, together with similar sayings of native origin, e.g. those of the Sibyl at Tibur (the 'Tiburtine Sibyl') of the brothers Marcius, and others, which had been circulating in private hands but which were called in, to be delivered to the Urban Praetor, private ownership of such works being declared illicit, and to be evaluated by the Quindecimviri, who then sorted them, retaining only those that appeared true to them.
From the Capitol they were transferred by Augustus as pontifex maximus in 12 BC, to the temple of Apollo Patrous on the Palatine, after they had been examined and copied; there they remained until about AD 405. According to the poet Rutilius Claudius Namatianus, the general Flavius Stilicho (died AD 408) burned them, as they were being used to attack his government.
Some supposedly genuine Sibylline verses are preserved in the Book of Marvels or Memorabilia of Phlegon of Tralles (2nd century AD). These represent an oracle, or a combination of two oracles, of seventy hexameters in all. They report the birth of an androgyne, and prescribe a long list of rituals and offerings to the gods. Their authenticity has been questioned. 
The Sibylline Oracles were quoted by the Roman-Jewish historian Josephus (late 1st century) as well as by numerous Christian writers of the second century, including Athenagoras of Athens who, in a letter addressed to Marcus Aurelius in ca. AD 176, quoted verbatim a section of the extant Oracles, in the midst of a lengthy series of other classical and pagan references such as Homer and Hesiod, stating several times that all these works should already be familiar to the Roman Emperor. Copies of the actual Sibylline Books (as reconstituted in 76 BC) were still in the Roman Temple at this time. The Oracles are nevertheless thought by modern scholars to be anonymous compilations that assumed their final form in the fifth century, after the Sibylline Books perished. They are a miscellaneous collection of Jewish and Christian portents of future disasters, that may illustrate the confusions about sibyls that were accumulating among Christians of Late Antiquity.
An incomplete list of consultations of the Sibylline Books recorded by historians: