Montanism was a movement begun by Montanus in the second century A.D., shortly after Montanus' conversion to Christianity. He claimed to have received a series of direct revelations from the Holy Spirit or the paraclete, and was joined by two women, Prisca and Maximilla. His preachings spread from his native Phrygia, where he proclaimed the village of Pepuza as the site of the New Jerusalem, across the contemporary Christian world, and threatened the church hierarchy. The beliefs of Montanism included the belief that the Trinity consisted of only a single person, similar to Sabellianism.

Although the mainstream Christian church prevailed against Montanism within a few generations, inscriptions in the Tembris valley of northern Phrygia, dated betweeen 249 and 279, openly proclaim their allegiance to Montanism. This sect persisted into the eighth century, although some people have drawn parallels between it and Pentecostalism. The most widely known Montanist was undoubtedly Tertullian, who is sometimes called the "Father of the Western Church".

The following text is a letter from Jerome to Marcella, in response to her question concerning Montanism. It is in the public domain, and is copied from http://www.ccel.org/fathers/NPNF2-06/letters/letter41.htm.

LETTER XLI
TO MARCELLA
An effort having been made to convert Marcella to Montanism, Jerome here summarizes for her its leading doctrines, which he contrasts with those of the Church. Written at Rome in AD 385

1. As regards the passages brought together from the gospel of John with which a certain votary of Montanus has assailed you, passages in which our Saviour promises that He will go to the Father, and that He will send the Paraclete -- as regards these, the Acts of the Apostles inform us both for what time the promises were made, and at what time they were actually fulfilled. Ten days had elapsed, we are told, from the Lord's ascension and fifty from His resurrection, when the Holy Spirit came down, and the tongues of the believers were cloven, so that each spoke every language. Then it was that, when certain persons of those who as yet believed not declared that the disciples were drunk with new wine, Peter standing in the midst of the apostles, and of all the concourse said: "Ye men of Judaea and all ye that dwell at Jerusalem, be this known unto you and hearken to my words: for these are not drunken as ye suppose, seeing it is but the third hour of the day. But this is that which was spoken of by the prophet Joel. And it shall come to pass in the last days, saith God, I will pour out of my spirit upon all flesh: and your sons and daughters shall prophesy, and your young men shall see visions, and your old men shall dream dreams: and on my servants, and on my handmaidens will pour out ... of my spirit."

2. If, then, the apostle Peter, upon whom the Lord has founded the Church, has expressly said that the prophecy and promise of the Lord were then and there fulfilled, how can we claim another fulfilment for ourselves? if the Montanists reply that Philip's four daughters prophesied at a later date, and that a prophet is mentioned named Agabus, and that in the partition of the spirit, prophets are spoken of as well as apostles, teachers and others, and that Paul himself prophesied many things concerning heresies still future, and the end of the world; we tell them that we do not so much reject prophecy--for this is attested by the passion of the Lord--as refuse to receive prophets whose utterances fail to accord with the Scriptures old and new.

3. In the first place we differ from the Montanists regarding the rule of faith. We distinguish the Father, the Son, and the Holy Spirit as three persons, but unite them as one substance. They, on the other hand, following the doctrine of Sabellius, force the Trinity into the narrow limits of a single personality. We, while we do not encourage them, yet allow second marriages, since Paul bids the younger widows to marry. They suppose a repetition of marriage a sin so awful that he who has committed it is to be regarded as an adulterer. We, according to the apostolic tradition (in which the whole world is at one with us), fast through one Lent yearly; whereas they keep three in the year as though three saviours had suffered. I do not mean, of course, that it is unlawful to fast at other times through the year--always excepting Pentecost -- only that while in Lent it is a duty of obligation, at other seasons it is a matter of choice. With us, again, the bishops occupy the place of the apostles, but with them a bishop ranks not first but third. For while they put first the patriarchs of Pepusa in Phrygia, and place next to these the ministers called stewards, the bishops are relegated to the third or almost the lowest rank. No doubt their object is to make their religion more pretentious by putting that last which we put first. Again they close the doors of the Church to almost every fault, whilst we read daily, "I desire the repentance of a sinner rather than his death," and "Shall they fall and not arise, saith the Lord," and once more "Return ye backsliding children and I will heal your backslidings." Their strictness does not prevent them from themselves committing grave sins, far from it; but there is this difference between us and them, that, whereas they in their self-righteousness blush to confess their faults, we do penance for ours, and so more readily gain pardon for them.

4. I pass over their sacraments of sin, made up as they are said to be, of sucking children subjected to a triumphant martyrdom. I prefer, I say, not to credit these; accusations of blood-shedding may well be false. But I must confute the open blasphemy of men who say that God first determined in the Old Testament to save the world by Moses and the prophets, but that finding Himself unable to fulfil His purpose He took to Himself a body of the Virgin, and preaching' under the form of the Son in Christ, underwent death for our salvation. Moreover that, when by these two steps He was unable to save the world, He last of all descended by the Holy Spirit upon Montanus and those demented women Prisca and Maximilia; and that thus the mutilated and emasculate Montanus possessed a fulness of knowledge such as was never claimed by Paul; for he was content to say, "We know in part, and we prophesy in part," and again, "Now we see through a glass darkly."

These are statements which require no refutation. To expose the infidelity of the Montanists is to triumph over it. Nor is it necessary that in so short a letter as this I should overthrow the several absurdities which they bring forward. You are well acquainted with the Scriptures; and, as I take it, you have written, not because you have been disturbed by their cavils, but only to learn my opinion about them.