Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Councils of Ancyra, Gangra, Neocæsarea, Antioch and Laodicea, which Canons were Accepted and Received by the Ecumenical Synods.: Canon VIII

Canon VIII.

Persons converted from the heresy of those who are called Phrygians, even should they be among those reputed by them as clergymen, and even should they be called the very chiefest, are with all care to be both instructed and baptized by the bishops and presbyters of the Church.

Notes.

Ancient Epitome of Canon VIII.

When Phrygians return they are to be baptized anew, even if among them they were reckoned clergymen.

Hefele.

This synod here declares the baptism of the Montanists invalid, while in the preceding canon it recognised as valid the baptism of the Novatians and Quartodecimans.  From this, it would appear that the Montanists were suspected of heresy with regard to the doctrine of the Trinity.  Some other authorities of the ancient Church, however, judged differently, and for a long time it was a question in the Church whether to consider the baptism of the Montanists valid or not.  Dionysius the Great of Alexandria was in favour of its validity:  but this Synod and the Second General Council rejected it as invalid, not to mention the Synod of Iconium (235), which declared all heretical baptism invalid.  This uncertainty of the ancient Church is accounted for thus:  (a) On one side the Montanists, and especially Tertullian, asserted that they held the same faith and sacraments, especially the same baptism (eadem lavacri sacramenta) as the Catholics.  St. Epiphanius concurred in this, and testified that the Montanists taught the same regarding the Father, the Son, and the Holy Ghost, as did the Catholic Church.  (b) Other Fathers, however, thought less favourably of them, and for this reason, that the Montanists often expressed themselves so ambiguously, that they might, nay, must be said completely to identify the Holy Ghost with Montanus.  Thus Tertullian in quoting expressions of Montanus, actually says:  “the Paraclete speaks” and therefore Firmilian, Cyril of Jerusalem, Basil the Great, and other Fathers, did in fact, reproach the Montanists with this identification, and consequently held their baptism to be invalid.  (c) Basil the Great goes to the greatest length in this direction in maintaining that the Montanists had baptized in the name of the Father, of the Son, and of Montanus and Priscilla.  But it is very probable, as Tillemont conjectured, that Basil only founded these strange stories of their manner of baptizing upon his assumption that they identified Montanus with the Holy Ghost; and, as Baronius maintains, it is equally “probable that the Montanists did not alter the form of baptism.  But, even admitting all this, their ambiguous expressions concerning Montanus and the Holy Ghost would alone have rendered it advisable to declare their baptism invalid.”  (d) Besides this, a considerable number of Montanists, namely, the school of Æschines, fell into Sabellianism, and thus their baptism was decidedly invalid.  (Vide Article in Wetzer and Welte Kirchenlexicon s.v. Montanus; by myself [i.e. Hefele]).

In conclusion, it must be observed that Balsamon and Zonaras rightly understood the words in our text, “even though they be called the very chiefest,” “though they be held in the highest esteem,” to refer to the most distinguished clergy and teachers of the Montanists.