9 These things being so, all who destroy the form of the Gospel are vain, unlearned, and also audacious; those, [I mean,] who represent the aspects of the Gospel as being either more in number than as aforesaid, or, on the other hand, fewer. The former class [do so], that they may seem to have discovered more than is of the truth; the latter, that they may set the dispensations of God aside. For Marcion, rejecting the entire Gospel, yea rather, cutting himself off from the Gospel, boasts that he has part in the [blessings of] the Gospel. Others, again (the Montanists), that they may 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; 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 those (the Encratitæ) who, on account of such as come in hypocrisy, hold themselves aloof from the communion of the brethren. We must conclude, moreover, that these men (the Montanists) can not admit the Apostle Paul either. For, in his Epistle to the Corinthians, he speaks expressly of prophetical gifts, and recognises men and women prophesying in the Church.
Sinning, therefore, in all these particulars, against the Spirit of God, they fall into the irremissible sin. But those who are from Valentinus, being, on the other hand, altogether reckless, while they put forth their own compositions, boast that they possess more Gospels than there really are. Indeed, they have arrived at such a pitch of audacity, as to entitle their comparatively recent writing “the Gospel of Truth,” though it agrees in nothing with the Gospels of the Apostles, so that they have really no Gospel which is not full of blasphemy.
For if what they have published is the Gospel of truth, and yet is totally unlike those which have been handed down to us from the apostles, any who please may learn, as is shown from the Scriptures themselves, that that which has been handed down from the apostles can no longer be reckoned the Gospel of truth. But that these Gospels alone are true and reliable, and admit neither an increase nor diminution of the aforesaid number, I have proved by so many and such [arguments].
For, since God made all things in due proportion and adaptation, it was fit also that the outward aspect of the Gospel should be well arranged and harmonized. The opinion of those men, therefore, who handed the Gospel down to us, having been investigated, from their very fountainheads, let us proceed also to the remaining apostles, and inquire into their doctrine with regard to God; then, in due course we shall listen to the very words of the Lord.
Source: Adversus haereses (New Advent)