There are two periods of church history which are particularly fascinating.
One is the period just after Jesus' death. The newly bolstered twelve, along with a certain convert from the Road to Damascus, start going around preaching and evangelizing. And yet, there's trouble at the mill. Some groups (including the Ebonites) are of the opinion that Christianity is really a Judaism 2.0, and therefore they're obliged to live as Jews but with different additional commandments. Paul wrote a scathing letter to the Romans warning of trusting in rules rather than faith and salvation. On the other hand, you had folks who thought that "so long as I don't hurt anyone, anything goes" and even the hedonists of their home town were appalled at their resulting scandalous behaviour, including marrying and having sex with one's own mother. Paul's letter to Corinth tried to sort that bunch out.
It was a "learn as you go" period, with Paul offering advice on everything from picking a Deacon to how women should act in Church, to some foundational ideas about what the nature of Christianity is.
But the second is the fifth century. Once the Emperor had okay'd the faith on his deathbed it started to get real, as people turned from what the nature of belief was to the nature of Christ.
The point's really settled now, with the whole concept of the Trinity, with a very orthodox belief about exactly what the nature of Godhead is - which is why priests and preachers don't punt on Trinity Sunday and give a sermon instead on how difficult it is to write a sermon about the Trinity because you're supposed to explain the Trinity on Trinity Sunday. And what might seem a minor debate on the finer points on a rather pointless avenue in philosophy by our standards was actually a huge thing back in the fifth century.
These days, few people really care that Bishop T.D. Jakes did or does advocate a heresy called modalism, originally advanced by Sabellius in the third century. Or that certain minority of a variant of Pentecostal churches still do - (claiming that God actually appeared in three forms but is really of one person). Or even, for that matter, that different denominations of Christianity even exist.
But back in the third to fifth centuries, it was seen as vitally important that the church be united and unified. Being the Body of Christ, it absolutely had to to have one belief, unity and orthodoxy, and to get it right. You couldn't have a working body if there were folks in the mix who believed Jesus only appeared to be human (Docetists), people who believed Christ was a human being "adopted" by God (Adoptionists) at either the baptism or the Resurrection, depending on the adoptionist. Heated and vigorous debate took place between these, the Gnostics who believed it was all about "secret magickal knowledge", and the main scrum-down between those advocating for a One Nature God, versus a Two Nature God.
To get into the minutiae of who argued what would be to get into Greek philosophy and a whole host of Christological terms like ουασις, and φυσιν and discuss various shades of meaning and splitting hairs between who was advocating what. The main upshot is that there were differing views and most tried to stay away from the two heretical boundary conditions - of Jesus being entirely God (advocates of which spat at opponents for suggesting such holiness could be created from the stinking effluent that flows from a woman's crotch once a month) or of Jesus being just another human being and prophet. So they rode a fine line. In fact, it got to the point where some people were getting confused and using terms interchangeably that actually meant crucially different concepts (personhood vs nature vs "species", basically).
What makes this interesting is that it was happening at a time when Imperial Rome was falling... when the tax base was being decimated by the wealthy avoiding taxes and the middle classes being increasingly unable to pay them. As a result, other cities and empires were rising, and the two main ones were Alexandria and Antioch. Constantinople eventually got into the mix as well.
So in addition to being pretty crucial in terms of having a unified body of believers, it was also a question of who was going to be the head of the new Empire, in a way. The church had power and influence and as secular imperial power waned, there were material and influence benefits to being with the church. So if it turned out that one set of theories held sway, Antioch would be the new seat of power. Alexandria would weild more influence if other theories became orthodoxy. Ephesus put in as being a key birthplace of various early Christians, and on it went.
Making a movie about this whole struggle would be impossible in some senses and rewarding in others. Impossible, because you really do need a whiteboard to keep track of the various terms and ideas being argued back and forth, and a chart to see who was winning which debate with which position. But rewarding, because it was an intersection of moral and church debate and concern, and material power. Emperesses and Queens got involved. Monks, who were not allowed to fight for personal honor or valor, were however mobilized as literal storm troopers to ransack opponents' churches, burn their buildings, and "take out" opposing people.
Nestorius was made Archbishop of Constantinople and took on the job even as he could see the writing on the wall that he was painting a large target on his back - nevertheless, he was accused of heresy, condemned, exiled and tossed with no defenders in the known world. Later, his theories would be seen as pretty much the orthodox position, with a couple of controversial tweaks. The man who went after him was also responsible for murdering a pagan patron of another prefect, Orestes - who had given him political backing.
Populations gathered under different colored banners to scream
slogans at each other, similar to a cross between a Crip/Blood argument
and a Tea Party rally, and predictable violence was the result. Dark
night meetings in catacombs, literal backstabbings, and vendettas
against people whose theories were discredited for religious or even for
secular purposes carried on for generations, taking out anyone who
associated with the "heretic" or anyone who started advocating a
position that even resembled a previous heresy. Cross "Game of
Thrones" (minus the sex) with The Sopranos (minus the crime), hose it
down with Dan Brown's Da Vinci Code type conspiracies and you're
starting to get there. A monk who attempted an assassination against a
key player in one of the camps was staked to a courtyard and tortured to death, and the other camp made him a Saint. One of the guys on the eventual winning side made a passionate defense of his position based on and promoting ideas and documents ultimately determined to be a forgery by a known heretic, but it was too late to change the outcome.
The whole thing came to a head with the Council of Chalcedon, in which the Two Nature camp won. The Western Churches accepted this, the Eastern did not. As a result, the Western churches grew in influence, and the Eastern did not. Christianity would be a vastly different faith if the Monophysite (One Nature) camp had won. Not only would the understanding of Christ be vastly different, but the seat of power would probably be in Antioch, not Rome. As it was, the Western empires increased and Antioch's influence faded, and the matter (from an orthodoxy standpoint) was mostly settled.
There were a few important things that came out of this turbulent period in human history. For starters, from a religious standpoint - ideas about something's nature and the relevance of the divine to the material shows up in terms of beliefs about consubstantiation and transubstantiation in the Eucharist - which is why Catholics, for example, believe that the Eucharist is transubstantiated, e.g. not a hypostatic union with bread, an amalgam of bread and Jesus, but the two natures of being bread and being the divine at the same time.
Schisms in the church resulted as a result of the struggle, with there being Chalcedonian churches (Catholicism, most Protestant churches) and Non-Chalcedonian (Coptic Orthodox, Syriac Orthodox). You've certainly heard of the former. Probably not the latter.
It also defined a vocabulary and a divide about which one could reference and argue - Calvinist and Lutheran thinking goes back and forth along this groove in later church history, as well as a Christological framework with which to argue other concepts, for example the nature of the Eucharist as mentioned before.
But to the victors go the spoils. The Western Chaledonian churches won the debate, won the battle, and won the empire. One would wane, one would wax. Supporters of one camp rose in power, the other empire's influence withered.
For a more in-depth look at the struggle, the book "Jesus Wars" by Philip Jenkins is a great primer, but one that needs to be read more than once to get all the characters and positions straight. But the most important and most fascinating aspect of the whole thing is the subtitle for that book - "How four Patriarchs, three Queens and two Emperors Decided what Christians would believe for the next 1,500 years."
The fate of Christianity, the whole framework of undestanding of the faith, and the resulting material power that flowed from this influence, was forged by only nine people - not by inspiration or peaceful debate - but in a period marked by bitter debate, outright murder, and thuggery.