Important Note from Schaff: Phrygia = Galatia During the Second Century Council of Ancyra

Original reference here:

Phrygia - A region of fluctuating boundaries occupying the central portion of Asia Minor. At the beginning of the Christian era the name had merely an ethnological and no geographical significance. There was no Roman province of the name Phrygia until the fourth century. In the northern part were the cities of Ancyra, Gordician, Doryleum; in the southern, Colossæ, Hierapolis, Laodicea. The region is of great importance for the history of religion after about 200 B.C., the cults of the West imported from the East receiving a profound impress from the primitive usages still current in Phrygia. Especially is this the case with the mysteries so strongly renascent in the century before the Christian era.

This helps put the pieces together for us and everything in its proper perspective.  The sect was originally called the 'New Prophesy' movement (and which is clearly related to Polycarp's eccentricities). It isn't called 'Montanism' until the fourth century. The process by which they became 'Phrygians' is a complex one.  Pseudo-Tertullian says that the Noetians are the ‘Kata Aeschinem’, who must be distinguished from the ‘Secundum Phrygas.' The Philosophumena assumes that the sect comprised of individuals of the Phrygian race.

The New Prophecy appeared in Phrygia either in 156—7 (Epiphanius) or 172 CE (Eusebius). The earlier date is to be preferred. Apollonius’ relates that Montanus “named Pepuza and Tymion (small towns in Phrygia) Jerusalem, in his desire to gather to them people from all quarters”. He can hardly have expected the New Jerusalem to come down in the Last Days on two towns at once, while, if Apollonius had mentioned the New Jerusalem, that ardent anti-Chiliast, Eusebius, would surely have seized on it, especially in H.E. 5,18,14.

Now we get to the interesting stuff.  If the original identification of the sect was racial rather than geographical (as the Philosophumena suggests) it is important to see that the First Council of Ancyra would have occurred both in the capital of the Roman province of Galatia (= modern Ankara) and the tradition region of Phrygia where many Phrygians continued to live.  From Wikipedia:

In 133 BC, the remnants of Phrygia passed to Rome. For purposes of provincial administration the Romans maintained a divided Phrygia, attaching the northeastern part to the province of Galatia and the western portion to the province of Asia. During the reforms of Diocletian, Phrygia was divided anew into two provinces: "Phrygia I" or Phrygia Salutaris, and Phrygia II or Pacatiana, both under the Diocese of Asia.

The religion of Phrygia was of course identified as the devotion to the Great Mother Cybele i.e. Magna Mater ("Great Mother"), or as Magna Mater deorum Idaea ("great Idaean mother of the gods"), equivalent to the Greek title Meter Theon Idaia ("Mother of the Gods, from Mount Ida"). Rome officially adopted her cult during the second Punic War (218 to 201 BCE), after dire prodigies, including a meteor shower and a failed harvest, seemed to warn of Rome's imminent defeat.  It was the prophesy of the so-called 'Phrygian Sibyl' - in reality the Hellespontine Sibyl - who prompted the nation to adopt the Phrygian goddess.

We can begin to intimate why Polycarp or his tradition was identified as Sibyllist or 'Sabellian' even though we should be very careful to avoid the mistake of Cybele = Sibylla made by many later authors.  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 same Hellespontine Sibyl who prompted rome to adopt Cybele.  The books were 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 (northwest of Naples in Italy) and from Cumae to Rome.

The story of the acquisition of the Sibylline Books by Tarquinius is one of the famous mythic 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. The Roman Senate kept tight control over the Sibylline Books; Sibylline Books were entrusted to the care of two patricians; after 367 BCE 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.

Charlesworth notes that the Christian Sibylline Oracles were written in Phrygia.  " At least the Jewish substratum of Sibylline Oracles l and 2 comes from Phrygia." This is indicated by l.l96-98, where Phrygia is said to be the first land to emerge after the Flood, and to become the nurse of restored humanity in the sixth generation. Also, in l.261f. , Ararat is located in Phrygia. The prominence thus given to Phrygia is the only indication of local provenance." I am not sure that there is a separate 'Jewish' and 'Christian' redaction. All we can say for certain is that there was a Phrygian interest in the Sibyl.