I ponder now and then the development of Christianity, which took place during the years the Roman Empire dominated most of Europe and the Mediterranean. I find fascinating the manner in which it also became dominant, and ascribe that in part to its remarkable assimilation of pagan religion and philosophy.
But I'm puzzled that in doing so it in turn became dominated by the doctrine of the Trinity. That seems to me a unique characteristic of the faith as it exists now, although it has not always been so.
There were many sons of gods wandering about in the first few centuries of what we now call the Common Era, being apparently uncomfortable with A.D. as a peculiarly Christian phrase. And there were gods who, like Christ, died for our salvation and were resurrected. Those other sons of gods, though, were considered human or semi-divine. Those gods were gods.
Christ for some reason is believed to have been while present on Earth both God and man, and indeed fully God and fully man. And, while he is indeed claimed to be the Son of God, he is also God the Father and the Holy Spirit or Ghost, who are also the Son of God, but are also, all of them one God.
If this seems to you a problem, it was also a problem during Christianity's development. Many thought Christ was divine but subordinate to the Father, a kind of sub-god, created by the Father. Some thought he was not always a god but became one. Some thought he was not a god, but despite that fact unique and extremely significant.
Unfortunately, because Christianity in most of its forms was exclusive and intolerant, the various kinds of Christians spent a great deal of time persecuting and even killing one another. Those who believed the Son to be one in Being with the Father, i.e. of the same substance and therefore in no respect subordinate, faced the problem that the Scriptures tended to indicate otherwise. Jesus in the gospels often seems to distinguish himself from the Father, and sometimes appears to claim that the Father knows something he doesn't know. I don't think anyone knew just what the Holy Spirit was supposed to be, really. Descriptions are quite vague. But for the fact he or it is God, we know very little. The Holy Spirit is a very nebulous God; one has to wonder why it is God at all.
Christianity's pagan predecessor religions avoided this confusion. The issue for them did not arise. And it seems early Christians didn't maintain that this was so because the pagan gods were imaginary or mere men. Instead, they thought them demons.
Constantine is generally credited with having made Christianity the religion of the Roman state, and it's claimed by some the doctrine of the Trinity was established and the condemnation of Arius and his followers (who felt Jesus was not of the same substance as the Father) was completed during his reign. But Constantine was rather eclectic and opportunistic when it came to religion. So were others of the time. We read of some individuals who kept statutes of Jesus, Asclepius, Apollo and others in their homes (there is something wonderfully Roman about this; a very practical people would have been careful to placate a variety of gods, keeping them all happy). He held a great Council of the Church, but there were many, many councils still to come, some of them not having imperial sanction. Constantine doesn't seem to have been concerned to impose Christianity or a particular kind of Christianity on the Empire. That came later.
Probably Theodosius was the emperor who began the imposition of the brand of Christianity we know best today, banning all others, also banning pagan worship and closing the schools of philosophy that were still active. The year in which the Western Empire disintegrated is traditionally considered to have been about 100 years later. The Eastern Empire lived on.
Augustine wrote of the Trinity, in Latin, and so is considered the great authority on the Trinity in Western Christianity, but some of the Fathers of the Eastern Church had written of it as well. They did so in Greek, though, which apparently Augustine could not read. We probably have Augustine and the decline of those would could read Greek to "thank" in great part for what we here in the West consider Christianity today. Augustine liked Paul, unfortunately, and Paul's grim view of the world and even grimmer view of sex was passed on to the Roman Church along with other things through Augustine.
Why was it so significant that Christ be one in Being with the Father? Why did that view win out, necessitating the creation of the convoluted, inexplicable Trinity? It would seem that the view that Christ, if he was a god, was a subordinate one would have made things far easier to explain and accept. Was it the fact that the emperors eventually accepted this view that made it dominant?
It must have been considered by some, at least, of great importance that Jesus be God and not "merely" the Son of God. But this required an explanation of his human existence. How could God be human without being diminished? That wouldn't be at all satisfactory. So, it had to be the case that he was both human and God, and also both Son and Father, regardless of the fact that this would seem impossible or nonsense.