Why did Tertulian became a Montanist?

There were two historical threads that contributed to and culminated in Tertullian becoming a Montanist.

Montanism and here was catholic and orthodox in its basic tenets, indeed it was Tertullian who outlined the word Trinity first, but it also looked for new or additional revelation via the Holy Spirit and to the purpose of the church body. Who forgives sin for example?

The first thread was how Rome, in Tertullian's view, had usurped what had been given to Peter alone. The keys to open the kingdom. Peter preached first to Jews and to Gentiles. The doors were open, never to be closed again. What had bound was loosed.

The second thread was similar in that the church had become a church of bishops, rather than of people of the Spirit.

I now inquire into your opinion, (to see) from what source you usurp this right to “the Church.”

If, because the Lord has said to Peter, “Upon this rock will I build My Church,”978 “to thee have I given the keys of the heavenly kingdom;”979 or, “Whatsoever thou shalt have bound or loosed in earth, shall be bound or loosed in the heavens,”980 you therefore presume that the power of binding and loosing has derived to you, that is, to every Church akin to Peter, what sort of man are you, subverting and wholly changing the manifest intention of the Lord, conferring (as that intention did) this (gift) personally upon Peter? “On thee,” He says, “will I build My Church;” and, “I will give to thee the keys,” not to the Church; and, “Whatsoever thou shalt have loosed or bound,” not what they shall have loosed or bound. For so withal the result teaches. In (Peter) himself the Church was reared; that is, through (Peter) himself; (Peter) himself essayed the key; you see what (key): “Men of Israel, let what I say sink into your ears: Jesus the Nazarene, a man destined by God for you,” and so forth.981 (Peter) himself, therefore, was the first to unbar, in Christ’s baptism, the entrance to the heavenly kingdom, in which (kingdom) are “loosed” the sins that were beforetime “bound;” and those which have not been “loosed” are “bound,” in accordance with true salvation; and Ananias he “bound” with the bond of death, and the weak in his feet he “absolved” from his defect of health. Moreover, in that dispute about the observance or non-observance of the Law, Peter was the first of all to be endued with the Spirit, and, after making preface touching the calling of the nations, to say, “And now why are ye tempting the Lord, concerning the imposition upon the brethren of a yoke which neither we nor our fathers were able to support? But however, through the grace of Jesus we believe that we shall be saved in the same way as they.”982 This sentence both “loosed” those parts of the law which were abandoned, and “bound” those which were reserved. Hence the power of loosing and of binding committed to Peter had nothing to do with the capital sins of believers; and if the Lord had given him a precept that he must grant pardon to a brother sinning against him even “seventy times sevenfold,” of course He would have commanded him to “bind”—that is, to “retain”983—nothing subsequently, unless perchance such (sins) as one may have committed against the Lord, not against a brother. For the forgiveness of (sins) committed in the case of a man is a prejudgment against the remission of sins against God.

What, now, (has this to do) with the Church, and your (church), indeed, Psychic? For, in accordance with the person of Peter, it is to spiritual men that this power will correspondently appertain, either to an apostle or else to a prophet. For the very Church itself is, properly and principally, the Spirit Himself, in whom is the Trinity of the One Divinity—Father, Son, and Holy Spirit.984 (The Spirit) combines that Church which the Lord has made to consist in “three.” And thus, from that time forward,985 every number (of persons) who may have combined together into this faith is accounted “a Church,” from the Author and Consecrator (of 100 the Church). And accordingly “the Church,” it is true, will forgive sins: but (it will be) the Church of the Spirit, by means of a spiritual man; not the Church which consists of a number of bishops. For the right and arbitrament is the Lord’s, not the servant’s; God’s Himself, not the priest’s. On Modesty

What is interesting is that while Rome rejected any new revelation and Tertullian, it subsequently embraced various new revelations apart from what was revealed in the Bible. Jude's contend for the faith once delivered.

So, Tertullian rejected the claims of Rome's two claims of authority and purpose. He embraced the workings of the Spirit instead.

To arrive at any considered view on this question, it is necessary to know a lot about Tertullian (circa 140 to 220 A.D.), and of the teachings of Montanus from Mysia, who was active under Antoninus Pius (ruled 138 to 161).

The teaching of Tertullian - He was a layman who lived in Carthage, North Africa, and he became the first notable representative of Latin-speaking Christianity, the first to coin the word 'trinitatus'. His contribution to clarity of expression about such foundational doctrines as the Trinity remains to this day. He attacked the persecution of Christians by the Emperors. In his later life, he swung at times to Montanism, more-so in later life. Here is a snippet from a scholarly work:

"Tertullian expressed the idea that Jesus Christ possesses two natures, and in so doing laid the foundation for the formulation subsequently adopted at Chalcedon... Tertullian, like Irenaeus, insisted that what the gospel tells us about the man Jesus must also be said of the Son as God. Thus he could speak of 'God crucified', and even say, 'God willed himself to be born'... Tertullian's language anticipates the explicit statements of Chalcedon... Tertullian wrote extensively against Marcion, but finished his career in the camp of the opposite heresy, that of Montanus." Heresies and Orthodoxy in the History of the Church, Harold O.J. Brown, pp 84-85 & 78, Baker Book House 1998

The teaching of Montanus - Again, I quote from the same source:

"Montanus was a 'charismatic', who maintained that he received direct revelation from the Holy Spirit. He considered himself the last great prophet, who would be immediately followed by the establishment of the heavenly Jerusalem. If one reason for the definition of the canon of Scripture was to make certain that nothing was improperly excluded - as had been done by Marcion - a second reason was to set limits to the crucial concept of authoritative, verbal divine revelation, and to make certain that biblical revelation was not diluted by the addition of spurious private revelations." (Ibid. pp 66 to 67)

Given all of that, why, indeed, did Tertullian end up in the 'camp' of the Montanists? Given the continuing statements of this source, it can be claimed that, in a nutshell, it was because of not heeding Jude's epistle to Christians in the first century, where he commanded them to "earnestly contend for the faith that was once delivered unto the saints" (Jude vs. 3). Further, it can be claimed that exactly that same danger has caused damage to the Church throughout the centuries, right to this 21st century, and we still see strong Christians being attracted to various charismatic leaders who claim extra-biblical, private revelations. Consider this analysis:

"The comparison between Marcionism and Montanism is enlightening, because the varieties of deviation they represent will continue to reappear in the church through the centuries. Marcion represents a movement that so radically transformed the Christian doctrine of God and Christ that it can hardly be said to be Christian, but rather a different religion using some Christian concepts, such as the .......... Motanism, by contrast, preserves most of the principles of orthodox theology, adding to them more recent, personal revelations of Montanus, the founder of the sect, and requiring a higher standard of asceticism than was usual in the rest of the church. A modern-day parallel appears to be offered by the ..........

Montanism was not really very distinctive theologically. This fact, together with the asceticism and the willingness of its adherents to endure martyrdom, made the Motanists particularly difficult opponents for the rest of the church. It opposed and attacked Gnosticism just as orthodoxy did, but by creating a separate organization on different principles, Montanus posed a similar danger. The Montanist movement was defeated almost incidentally as a result of the measures the church took against Gnosticism and Marcionism, especially by the definition of the canon of Scripture, which closed the door to Montanus' new revelations." (Ibid. pp 67-68)

Therefore, the reasons why Tertullian was drawn aside, as it were (spiritually speaking) are exactly the same reasons we see this going on with individuals to this very day: forgetting to heed Jude's warning to stick only to the faith that was revealed to the first century saints.