As a theologian I very rarely have anything to contribute to this sub, but here's my chance!

While u/MuskyElongatedRocket 's answer is a fascinating look into the possible roots of trinitarian thought, I am fairly certain that the main thrust of his argument with regard to your question does not reflect mainstream scholarship - that early Christians viewed themselves as torch bearers of the Essenes, and that this is what made trinity important to them. Though the similarities are remarkable, the arguments for the jump from similarities to influence and importance are not there. It would require:

1 - That we take Josephus', Philo's and Pliny's descriptions of the Essenes at face value, i.e. that the Essenes were a real and widely known sect with tremendous influence. The historical reliability of their accounts regarding the Essenes is debated (notably by Steve Mason), although it seems likely that the sect did actually exist. However, the ancient sources certainly do not describe their theology very well and they certainly do not name any texts (such as Enoch) that they considered authoritative.

However, we can speculate on their theology if we identify the Qumran sect that collected (and possibly produced some of) the Dead Sea scrolls as an offshoot of the Essenes (or vice versa, as the Jewish Virtual Library does). While this identification remains the most widely accepted hypothesis (though notably challenged by, again, Steve Mason, and others), the theology of the Qumran Sect and how it relates to wider Essenism is not clear cut.

2 - That the Essenes' influence carried over all the way to the 3rd and 4th century Christians who formulated christology and ultimately trinitarianism, which is a terribly unsubstantiated claim, especially considering the problems above. It seems all the more unlikely when we take into account that the earliest Christians seem to have been quite unconcerned with the exact nature of Christ and the Holy Spirit with regard to the Father. Those questions only appear in the following centuries long after the Essene sect had died out.

I will try to answer your question from a theological perspective. The doctrine of the Trinity can only be understood in the context of the christological controversies of the 3rd and 4th centuries, and the crux of those controversies lies in soteriology - i.e. who exactly was Jesus of Nazareth, and how exactly does his death and resurrection result in the salvation of souls.

The First Council of Nicaea in 325 formulated the Nicene Creed that declares belief in a Christ that is "begotten, not made, being of one substance with the Father." This was a reaction especially to Arianism that considered Jesus to be just an ordinary man. Notably, concerning the Holy Spirit, the original creed just states belief in it but does not elaborate any further. It's not considered important at this point. Instead the creed proceeds to condemn some Arianistic beliefs about Christ such as "there was a time when he was not".

Edit: Arians absolutely did not consider Jesus an ordinary man. The distinction to orthodox belief was more subtle, concerning whether the Son of God, although divine, was someone who came to be as a willful act of creation by the Father at some point in time, something that the Nicene creed explicitly rejects.

Probably the most important single theologian to the formulation of christology is Athanasius of Alexandria. He opposed Arianism fiercely, and with regard to why the distinction to Arianism and others was thought to be so important, he points out that creation can not be saved by something that is part of it. Furhermore, Christ being anything less than fully divine would mean the Church being guilty of idolatry.

Again, if the Son were a creature, man had remained mortal as before, not being joined to God; for a creature had not joined creatures to God, as seeking itself one to join it; nor would a portion of the creation have been the creation's salvation, as needing salvation itself. To provide against this also, He sends His own Son, and He becomes Son of Man, by taking created flesh; that, since all were under sentence of death, He, being other than them all, might Himself for all offer to death His own body; and that henceforth, as if all had died through Him, the word of that sentence might be accomplished, and all through Him might thereupon become free from sin and from the curse which came upon it, and might truly abide for ever, risen from the dead and clothed in immortality and incorruption. (Orations Against Arians II, 69)

While Arianism was no doubt the most wide-spread alternative to what eventually became the orthodox belief, all other interpretations were condemned for the same reason: they were considered incompatible with salvation in Christ.

After the relationship of the Son and the Father was settled, it seems natural that Christians followed up with the question of where the Holy Spirit fits in all this. While it was agreed relatively quickly during the latter part of the 4th century that the Spirit must also be divine and equal to the Father and the Son, the exact relationships between the persons of the Trinity caused some debate (notably the filioque-controversy). However, none of it was nearly as important as the question of the Son's relationship to the Father.

Alister E. McGrath's Christian Theology: An Introduction provides an excellent overview of the development of early Christian doctrine.

On my criticism of the Essene argument I cite Cecilia Wassén's Qumran-bevegelsens opprinnelse og identitet (transl. the origin and identity of the Qumran movement) in Dødehavsrullerene: Deras innhold, historie og betydning (2009), which unfortunately is not available in English, but it is the only thing I had access to right now.

Edits for formatting, spelling and clarifications.

Edit: I missed your edit about not being interested in theology. Sorry about that. Regardless, I hope this explains why it was such an important issue.

Ok, I've got to set you up some context first. You're probably aware of the four main sects of Judaism in Jesus's day: Pharisees, Sadducees, Essenes, and Zealots. Regardless, a quick rundown of their politics:

Pharisees: Puritanical sorts that were most concerned with Jewish Law

Sadducees: Greek Jews who wished to synchronize Judaism to the broader Grecco-Roman philosophy and theology.

Essenes: Cryptic esoteric sorts but very secretive

Zealots: Something akin to Jihadists today.

The Jewish Virtual Library has a lovely exploration if you'd like to read more.

Now, onto your question. At some point during the Maccabean rebellions, there was a split between the Essene and the rest. It's believed the Essenes took great offense to the policies of the Pharisees and Sadducees for often integrating pagan things into the religion. The Sadducees integrated Pagan practices, while the Pharisees decided to primarily use Greek as the language and Hebrew began to die out among the scholars and elite of Palestine. The aforementioned Jewish Library states:

Jonathan (160-142 B.C.E.) and Simeon (142-134 B.C.E.) usurped the office of high priest (which included secular duties), much to the consternation of conservative Jews; some of them could not tolerate the situation and denounced the new rulers.

This split, I want to argue, is the fundamental origin of this psychological argumentative style of Early Christians two centuries later. I personally believe, and hope my evidence shows, that these Essenes in many ways form something of a foundation for Christian theology to build off of.

It might surprise you to learn, but there's some evidence that the very concept of a trinity has roots from the Essenes. Between the Maccabees to Jesus, there developed an idea of a "lesser" YHWH and a "greater" YHWH among the Essenes. Andrei A. Orlov, professor of Judaism and Christianity in Antiquity at Marquette University, has some writing on this to get you introduced. That can be found here. He makes mention of the development of a unique "Enochian" theology. He quotes Christopher Morray-Jones, an author on this topic, as so:

As the Angel of the LORD, Metatron functions as the celestial vice-regent who ministers before the Throne, supervises the celestial liturgy and officiates over the heavenly hosts. He sits on the throne which is a replica of the Throne of Glory and wears a glorious robe like that of God. He functions as the agent of God in the creation, acts as intermediary between heavenly and lower worlds, is the guide of the ascending visionary, and reveals the celestial secrets to mankind. He is, by delegating divine authority, the ruler and the judge of the world. He is thus a Logos figure and an embodiment of the divine Glory. In his shi(ur qomah, we are told that Metatron’s body, like the kabod, fills the entire world, though the writer is careful to maintain a distinction between Metatron and the Glory of God Himself

It's this "Enochian" theology that I wish to focus on. This Metatron character, although never named in Enoch, has some descriptions which I would argue are very clearly sources for Early Christian ideology.

Let me grab a section of Chapter 48 of Enoch and ask you if this sounds like Jesus:

48:3 At that hour, that Son of Man was given a name, in the presence of YHWH of Hosts, the Before Time; even before the creation of the sun and the moon, before the creation of the stars, He was given a name in the presence of YHWH of Hosts.

48:4 He will become a staff for the righteous ones in order that they may lean on Him and not fall. He is the Light of the gentiles and He will become the hope of those who are sick in their hearts.

48:5 All those who dwell upon the earth shall fall and worship before Him; they shall magnify, bless, and sing the NAME of YHWH of Hosts.

48:6 For this purpose He became the Chosen One; He was concealed in the presence of YHWH of Hosts prior to the creation of the world, and for eternity.

48:7 And He has revealed the wisdom of YHWH of Hosts to the righteous and the kodesh ones, for He has preserved the portion of the righteous because they have hated and despised this world of oppression together with all its ways of life and its habits in the NAME of YHWH of Hosts; and because they will be saved in His Name and it is His good pleasure that they have life.

Here we can identify several characteristics:

• He is from before Creation
• He is the destined Messiah (chosen one)
• He will be for the Gentiles
• People worship him as if he is God.

Doesn't it sound rather trinitarian? Albeit in this case, it's more duolitarian but still...

Now, here's the surprising part. This text might date to about a century before Jesus ever came. That's 270 years before the use of Trinity to be used to describe God in Theophilus of Antioch's work. It's remarkably curious to find this explicit description of a multi-person singular Godhead before Jesus was here.

But again, I want to emphasize it might be older than Jesus. As far as I'm aware, not complete versions can be dated to that. But many fragments can. However all the same, it is still definitely older than Christian councils on defining the trinity.

So, this is what I am arguing: the reason why Early Christians were so passionate on the issue of the trinity, is because Christianity itself was positioning itself to be the inheritors of the Essene's heritage. They seemed to be actively building on and inheriting the political situation of the Essenes. For early Christians, the story of the Essenes must have felt very relatable to their own experiences with mainstream Judaism. Enoch is quoted in the New Testament, and acknowledged by some early Christians as worth exploration. So there's a powerful argument to be made that Christians were absorbing Enochian theology and with it it's disgust of mainstream Judaism's view of a singular YHWH.

I think early Christians viewed themselves as the Essene torch bearers. So they began writing and expressing themselves in the same way Essenes did. I'd go so far to argue that the word itself Essene might be related to "The Way", what Christians called themselves before they were called Christians: The Aramaic word Essaioi can be said to mean the "Pious Followers."...

I hope this might aid you in your thoughts and perhaps add context to why they were so passionate on that issue. For them, the Essene texts were older than Jesus but pointed to their theology. It's a helluva thing to be passionate of when you win the historicity argument.

Edit: formatting, spelling.