Can I correct your post a bit. I'm pretty familiar with the Arian controversy, as it's the background to the very interesting fact that pretty much all the modern churches that quote the Apostle's creed (the modern version of the Nicene Creed) don't believe it, at least not as it was originally agreed upon. (Reference is Eusebius' letter explaining the thought of Constantine and the council upon the creed, which was an adjustment to Caesarea's, Eusebius' church, rule of faith, as quoted in Socrates' Ecclesiastical History.)
Christ is called God very, very early, even in NT writings (we can debate why and what was meant, but we can't debate that he's called God). Ignatius, in letters that are not at question, refers to Christ as God. The letter of Pliny the elder (I don't know the date) says that the Christians used to sing hymns to Christ as God. Justin Martyr, around AD 150, has some very, very thorough explanations of his view of Christ as God.
In the end, though, Tertullian has a summation of Christ as God around AD 200 (Against Praxeas) that is excellent, and it explains the usage found in the NT, in Justin's writings, and in everything else seen in the early fathers extremely well.
I say all that, because when you get to Nicea and to Eusebius' letter as quoted in Socrates' history (not the philosopher Socrates), Eusebius' letter lines exactly up with what Tertullian and Justin said as well as with the Nicene Creed.
Sorry this is a little long. It's very, very interesting to me, and I'm setting up my comment about Arius.
Nicea did not end a fight over the Trinity; it heated it up. Arius really started the fight, but here's why it was a fight.
Before Nicea, the early churches (non-gnostic) regularly quoted passages like Prov 8:22, saying that Christ was created. They did not distinguish between his being born and being created. You'll find all through the writings of the fathers statements that Christ "the Word" had an inexplicable, incomprehensible birth in the beginning. "The Word," which was part of God, was birthed by God from out of himself. They really liked to quote Ps 45:1, "My heart has emitted a good word," as a proof text.
Okay, I said all that to say that what Arius said was that this birth was a real beginning for Christ. Orthodox believers--those who would agree with Tertullian, Origen, Athenagoras, and pretty much all the fathers who speak on the matter--would have said that the Word existed before his birth, but was inside of God rather than "emitted." (Tertullian explains all this thoroughly, but so do Justin, Athenagors, and Origen, who added some twist on "eternal" so that there was no beginning.)
Orthodox believers (please excuse my terminology, I just mean those who agree with the writers I'm mentioning) really didn't care much about Arius's adjustment. They didn't agree, but it didn't matter much. Bishop Hosius, for example, a respected bishop from Spain, wrote a letter telling all parties involved to just drop it.
They didn't drop it, however, because the real problem is that there were always a lot of modalist believers in the "orthodox" church. Such believers did not distinguish between "persons in the Godhead." They believed that the Father is Jesus is the Holy Spirit. There's only one person.
Arius's bishop, Alexander, was more modalist than orthodox. He was really, really irritated with Arius's view, and bad blood developed between them. Alexander had power, as bishop, so Arius took his argument to the market place, creating jingles to be sung by children. (This is all very embarrassing, as I would consider myself as belonging to the lineage of these "orthodox" churches.)
Alexander got people on his side, and Arius got people on his side. However, the "orthodox" view was really in between the two. After Nicea, you can tell from Eusebius's letter that he's scared that the modalist side, Alexander's side, may have won out too much. He is very apologetic about the Nicean decision, and he explains in a very defensive way to his congregation why he agreed with it. (As a result, and also because there was an Arian named Eusebius from Nicodemia, he is often labelled an Arian or accused of having Arian tendencies, when in fact he is extremely "orthodox.")
However, the Nicene creed is really very orthodox, and agrees quite nicely with Tertullian's view, and that of the other apologists. There is one God, it says, and that one God is the Father. That one God has a Son, who was begotten of his own substance (this wording is found way back in Athenagoras around AD 170, so it's not new). Thus, the Son was made from "God stuff," the "divine matter," rather than from "mere matter" like everything else, including the angels.
Since the Son is made from "God stuff," then he is "God from God, light from light, very God from very God."
All very typically "orthodox," except that they now backed off from using the word created, because that helped Arius too much. Nicea made only that one small change from the "orthodox" view.
In the end, though, there was another group, that Tertullian said always constituted the majority of the believers (because they were ignorant and uneducated, he said). That was the modalists, and there was a lot of them, and they opposed the Arians most strongly. They had to be appeased.
As I said, the battle only heated up after Nicea, with the emperors getting involved, appointing Arian or anti-Arian bishops depending on his current view. Eventually, the "orthodox" and the modalists got mixed together, thanks to the immense charisma of Athanasius, the leader of the "Niceans."
By the latter half of the 4th century, three or four decades after Nicea, the "orthodox" view was lost. It merged into the modalist view and disappeared. The modalist view lost most of its appeal, too, because now the orthodox/modalist mixture was good enough for them. In that one, there is one God, but the one God is not the Father with a Son that is his divine Word. The one God was now the Father, Son, and Holy Spirit together, three in one.
All that should be obvious. After all, the first words of the Nicene Creed are "We believe in one God, the Father." That is not the belief of modern believers who quote that creed. They "believe in one God, the Father, Son, and Holy Spirit."
That happened during the fourth century. It was a fight between leaders of the orthodox/catholic churches (not Roman churches, as there was no Roman church except in Rome at that time; Nicea appointed four patriarchs, not one, and they appointed four, because there were none before that.) It was not the leaders of anything using a doctrine to maintain their power.
Conspiracy theories are always popular, because we humans tend to think that way. That whole story of Arius looks like natural evolution, not any sort of purposeful anything.
The apologists who led the fight against the other Christianities from 150 to 300 (that Equinox mentioned) didn't need a specific Trinity doctrine to disagree with the other Christianities. It seems strange to me equinox said that, because he's read a lot of those things. The disagreements on who Christ was were plenty great enough without the apologists having to invent something to fight with the Valentinians, Marcians, etc. In fact, I really object to the suggestion that there was a development. Justin, Tertullian, Origen, and Athenagoras have very little difference between them (except Justin's rather bold wording and Origen's eternal begetting), and there is absolutely no difference between Tertullian's descriptions of the Trinity and Eusebius' letter about the Nicene creed. Those are so similar and use such similar texts, you'd think Eusebius was trained by Tertullian, and since he had read him, maybe he was. Athenagoras, too, though explains the Trinity pretty thoroughly and in exact accordance with Nicea, all the way down to explaining the issue concerning the substance of God, the "material" God is made of, and Athenagoras is from over a century before Nicea.
Again, I'm sorry this is so long, but it's one of my favorite subjects.
How then was one to understand the relationship of Yeshua of Nazareth to his God?
They got over their strict monotheism pretty handily. Justin says there's two Gods, one begotten and one unbegotten, and Tertullian questions Praxeas' math, saying that in John 1, if there's a God and there's a Word that's with God and who is divine, then anyone who can add can count two in that passage. They didn't sweat the use of the word two, even while they said that the Father was the one God.
Tertullian had a great explanation of why that was. He said that people call a sunbeam the sun when it's shining in the window, but if they talk about the sun and the sunbeam at the same time, then they "immediately remove the name of sun from the mere beam, so it is with the Word and God." He explains that the Word is worthy of the title God, but if you are talking about God and his Word, then you no longer refer to the Word as God, but as Lord.
This fits really well with what Paul wrote, and Tertullian said his description was how the apostles taught the churches and their elders verbally.
™ Version 4.2