According to the testimony of the early church fathers Irenaeus, Tertullian, Hippolytus, and Eusebius; Montanus wrote the book "The New Prophecy" in which he taught the following:

• When Jesus spoke about sending another comforter he meant in addition to the fist outpouring of the Holy Spirit, and Montanus himself was the "other" comforter. It is unclear if He and Maxumilla were holy spirits with their own power (Similar to the Way cult) or were infilled with The Holy Spirit (like modern day Pentecostals) 
• His second outpouring of the Holy Spirit consisted of false prophecies and a "vain babbling" that was never practiced by any Old Testament or New Testament prophet or teacher. 
• He also taught the LORD would soon return and set up the kingdom in his hometown of Phrygia (like the RLDS cult teaches it will be Independence Missouri) 
• When his holy spirit would come over a person it would be in what he called an "Ecstasy" Tertullian describes this "ecstasy" as the same as when the power of God over shadowed Adam when he fell into a deep sleep. (Similar to the "Slain in the Spirit" phenomenon) Eusebius on the other hand describes the "Ecstasy" as falling down and thrashing about.
• Some of the Phrygians broke from Montannus' group to form the CataPhygians. They were Montanists that also taught the Praxain heresy - That Jesus is the Father. (similar to the Oneness Pentecostals) 

Irenaeus 178

• Against Heresies 3.11 - Montanists set at nought the gift of the Spirit, which in the latter times has been, by the good pleasure of the Father, poured out upon the human race, do not admit that aspect of the evangelical dispensation presented by John's Gospel, in which the Lord promised that He would send the Paraclete (John 16); but set aside at once both the Gospel and the prophetic Spirit. Wretched men indeed! who wish to be pseudo-prophets, forsooth, but who set aside the gift of prophecy from the Church; acting like the Encratitae who, on account of such as come in hypocrisy, hold themselves aloof from the communion of the brethren. We must conclude, moreover, that the Montanists can not admit the Apostle Paul either. For, in his Epistle to the Corinthians, he speaks expressly of prophetical gifts, and recognizes men and women prophesying in the Church. Sinning, therefore, in all these particulars, against the Spirit of God.

Tertullian

• Marcion 4.22 - The Montanist book "The New Prophecy" teaches when God speaks through man a "grace ecstasy or rapture" is imparted whereby "he necessarily loses his sensation because he is overshadowed with the power of God"
• Marcion 5.8 - He agrees with Montanus that gifts of the Spirit accompany "ecstasy or rapture" as it is called.
• Praxeas 1.1 - Praxean heresy is that Jesus is the Father incarnate. After the Bishop of Rome had acknowledged the prophetic gifts of Montanus, Prisca, and Maximilla. Praxeas taught his heresy in conjunction with Montanus ideas. This lead to the bishop of Rome recanting Montanus and praxean ideas. (Tertullian was a Montanist at this time.)
• Heresies 1.7 - Cataphrygians - the holy spirit was in the apostles but the Paraclete was only in Montanus and he is superior to Christ. Some of the Cataphrygians (called Cataproclans or Cataschinetans) also teach Jesus is the Father.
• Heresies 1.9 - Praxeas introduced a heresy which Pope Victor(inus) was careful to corroborate, that  Jesus Christ is the Father.
• Praxeas 1.1 - Praxean heresy is that Jesus is the Father incarnate. After the Bishop of Rome had acknowledged the prophetic gifts of Montanus, Prisca, and Maximilla. Praxeas taught his heresy in conjunction with Montanus ideas. This lead to the bishop of Rome recanting Montanus and praxean ideas. (Tertullian was a Montanist at this time.)
• Treatise of the Soul 1.45 - Thus in the very beginning sleep was inaugurated by ecstasy: "And God sent an ecstasy upon Adam, and he slept."
• On fasting - It is these which raise controversy with the Paraclete; it is on this account that the New Prophecies are rejected: not that Montanus and Priscilla and Maximilla preach another God, nor that they disjoin Jesus Christ (from God), nor that they overturn any particular rule of faith or hope, but that they plainly teach more frequent fasting than marrying. Concerning the limit of marrying, we have already published a defence of monogamy.(3) Now our battle is the battle of the secondary (or rather the primary) continence, in regard of the chastisement of diet. They charge us with keeping fasts of our own; with prolonging our Stations generally into the evening; with observing xerophagies likewise, keeping our food unmoistened by any flesh, and by any juiciness, and by any kind of specially succulent fruit; and with not eating or drinking anything with a winey flavour; also with abstinence from the bath, congruent with our dry diet. They are therefore constantly reproaching us with Novelty; concerning the unlawfulness of which they lay down a prescriptive rule, that either it must be adjudged heresy, if (the point in dispute) is a human presumption; or else pronounced pseudo-prophecy, if it is a spiritual declaration; provided that, either way, we who reclaim hear (sentence of) anathema.

Hippolytus

• The Refutation of all Heresies 8.12 & 10.22 - Montanists false prophets, introduce novelties of fasts and feasts. Some teach the father is the son.

Eusebius

• Ecclesiastical History 5.16-17 - In describing the Phrygian heresy, Eusebius says: "Montanus became beside himself, and being suddenly in a sort of frenzy and ecstasy, he raved, and began to babble and utter strange things, prophesying in a manner contrary to the constant custom of the Church handed down by tradition from the beginning. Some of those who heard his spurious utterances at that time were indignant, and they rebuked him as one that was possessed, and that was under the control of a demon, and was led by a deceitful spirit, and was distracting the multitude; and they forbade him to talk, remembering the distinction drawn by the Lord and his warning to guard watchfully against the coming of false prophets.
  But others imagining themselves possessed of the Holy Spirit and of a prophetic gift, were elated and not a little puffed up; and forgetting the distinction of the Lord, they challenged the mad and insidious and seducing spirit, and were cheated and deceived by him. In consequence of this, he could no longer be held in check, so as to keep silence. Thus by artifice, or rather by such a system of wicked craft, the devil, devising destruction for the disobedient, and being unworthily honored by them, secretly excited and inflamed their understandings which had already become estranged from the true faith."

  He goes on to mention that the leaders of the church after studying the "babbling spirit" and the prophecies that did not come to pass judged it to be the work of a "false and seducing spirit" and they separate themselves from the heretics and withhold communion from them also.

  To prove that a true prophet does not "speak in ecstasy" he says, in chapter 17:
  "The false prophet falls into an ecstasy" which is "purposed ignorance" or "involuntary madness of soul" "But they cannot show that one of the old or one of the new prophets was thus carried away in spirit." Not "Agabus, or Judas, or Silas, or the daughters of Philip, or Ammia in Philadelphia, or Quadratus, or any others."

  He also stated if they were real prophets, their predictions would be 100% accurate because "the apostle thought it necessary that the prophetic gift should continue in all the Church until the final coming. But they (Monanists) cannot show it"

Asterius Urbanus 300

• 3.2 - The “maddening spirit” caused Montanus to hang himself. A few years later It caused Maximillia to commit suicide in the same way.