The underlying basis for what eventually became the Trinity concept did exist as a tentative or nebulous idea in the heads of some early Christian writers, but was never laid out specifically until the middle of the 4th century. This was the practical start of the Trinity doctrine. I will, however, begin with the theoretical origins of the Trinity, then move on to its practical origins (i.e. when it became a part of Christianity’s central theology).
Really, though, even this is not novel; the gospels speak of the Father, Christ, and Paraclete, and these were commonly interpreted as God, Word, and Wisdom. In fact, more mystical versions of Christianity, especially some Gnostic sects, had cosmologies in which there was an Ineffable Divine, from whom Christ and Wisdom both emanated (along with many other emanations, too).
Other Church Fathers refer to the “divine triad,” if you will. Among the most important of these was Tertullian, who — because he wrote in Latin — provided the origin of the English word "trinity" (i.e. his Latin trinitas). Given the scriptures they had before them, the Church Fathers hardly could have failed to do so. Even so, they didn’t develop this into a doctrinal or theological point; it appears they saw no need for any such thing, that the mentions of the three divine Persons in scripture were self-sufficient.
What Nicaea left out, of course, was the Holy Spirit. The controversy leading up to that Council was, rather specifically, the nature of Christ and his relationship to God the Father. The Holy Spirit’s nature was not addressed; literally, then, Nicaea could not have produced a “Trinity”; at best, it produced only a “duality.”
Developing the theology needed to back up this belief, fell to a number of Church leaders, perhaps the most important being Athanasius of Alexandria (and, while he lived, Athanasius’ mentor, Patriarch Alexander). As a way of bolstering their theology, they took up the matter of the Holy Spirit’s divinity, as well. The essential foundation of this doctrine was threefold:
One complication that early Trinitarians faced, was that their doctrine was so irrational, alien, and illogical, that the very tool needed in order to explain it, e.g. language, was quite simply not up to the task of describing it. They were forced to coin new words, such as ‘ομοουσιος (homoousios), meaning "one substance." This, of course, didn't help much, since words like homoousios didn‘t mean very much to the reader/listener, unless s/he already had some inkling of what the Trinity was supposed to be. One could understand what the Threee Persons of the Godhead having "one substance" meant, only if one already had some idea that they were somehow joined or linked as one ... in which case, being told they had "one substance" no longer was very informative.
The various points of evidence came from the apostolic gospels (mentioned above) only gave the Trinity greater apparent authority — at least, in the eyes of those who supported it. Opponents used other scriptures (e.g. “the Father is greater than I,” Jn 14:28) to attack it, and they considered their evidence just as authoritative.
A point which cannot be missed here is that, in the early 4th century when the Trinity was first thought out, it was not the intended goal. Instead, the goal was to support a christology (in Athanasius’ case, the orthodox or Alexandrine; in the Arians’ case, the Samosatene/Arian doctrine). Attempting to understand the nature of God was actually secondary to the real effort, which was to bash one's theological opponents.
Athanasius had a turbulent career, wrestling not only with his own see of Alexandria (which, for several generations after Nicaea, remained home to many Arians, who fought with him and even expelled him a couple of times), but with Christian thinkers in other regions who either opposed him, or else supported his orthodoxy but wished to go about it differently. The idea of a mysterious, inexplicable divine Trinity wasn’t without its critics, in the 4th century.
Athanasius ultimately forced his doctrine on his diocese and evangelized for it elsewhere. At the First Council of Constantinople in 381, the matter was taken up. Even prior to the invocation of the Council, Athanasius intimidated many of the bishops present; their adoption of the Trinity as one of that council’s canons might well be attributed to that.
This Council enacted a creed which is, more or less, what is now known as the Nicene Creed. Properly speaking, it’s not the “Nicene” creed at all, and rather should be called the Constantinopolitan Creed. But it was called the New Nicene creed at the time, as a way of affirming all that had been done at Nicaea.
This has, ultimately, ended up being the position of most of Christendom: That the Trinity is true, even though it’s incomprehensible; faith is the only tool one can use to accept it.
That the Trinity doctrine was constructed not out of a desire to understand God, but rather as part of a a vendetta against heresies (i.e. first Arianism, then Macedonianism), is what makes it as nonsensical as it seems. It was not designed as a rational theology and was never intended to be one; it was designed specifically, and only, to refute other theologies and to exclude those who adhered to them.
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