Previously on "YHWH's Excellent Adventure"
Part 1: Thus Spoke Zarathustra. In which we learn that YHWH wasn't the first god to want to be alone.
Part 2: YHWH rules, Chemosh drools! In which we learn most of Biblical history was pulled out of King Josiah's backside.
Part 3: By the Rivers of Babylon. In which the Hebrews crib most of their afterlife from the Persians.
Part 4: Alex the Kid in Hellenic World. In which Alexander the Great conquers the known world and everyone learns to love, Greek-style.
Interlude: Danny Boy, the Lions, the Lions are calling. In which Daniel becomes a Prophet despite not knowing the first things about his own surroundings.
Part 5: I ♥ MACCABEES. In which the flame of cosmopolitan Hellenism is snuffed out by religious fundamentalists.
Part 6: Quacks, Kooks and Loons of the Roman Empire. In which we see that the early Roman Empire was a utopia for two bit religious frauds and crazy cults.
Part 7: Would the real Messiah Please Stand Up? - In which we began to example just how much we really know about Jesus.
Part 8: The Greatest Story Ever Made Up - In which we see as the four gospels progressively transform Jesus from Jewish messiah and adopted son of god into the ever-co-eternal Logos and saviour of mankind
Part 9: Speaking in Tongues - In which early Christianity is finally torn from its Jewish roots, and gentiles steer the religion to a far different future than its founders intended.
Part 10: I, Constantine - In which Christianity is Romanized, and becomes the state religion of one of the greatest powers of the world.
Up next...
Part 11: You say homoousios, I say Homoiousios
The path to modern orthodoxy was never going to be a straightforward one, and, modern Christian theology was just one of many ideas on offer as the religion matured. However it is mistaken to say that modern Christianity's ideas were "wrong"and that early Christianity was "correct", or vice-versa. As with all things, it is never as simple as that, and merely brings in a false sense of dichotomy for something that is much more fragmented.
One of the most fraught parts of the developing theology is what is termed "Christology", or specifically theology that concerns Jesus in particular.

Above: This is the first result of a Google image search for "Christology". No, I have no idea what it is supposed to mean either.
As we learned in previous installments, as Christianity became increasingly Hellenized as a religion, Jesus transformed from a a mortal priest-king of the Davidic line, to a divine virgin-born son of God. Some sects, particularly in the Middle East, continued to believe in the mortal ideas, but these became heavily marginalized, particularly after the Roman destruction of Jerusalem.
Once Jesus was known to be divine, then further questions came along. And this is where it starts to take on a silliness not to be seen until thousands of years with and debates amongst Star Wars fans about the length of Darth Vader's star destroyer.
Most of the back and forth over heresy vs. orthodoxy really only began to pick up steam after the Council of Nicaea in 325 AD, prior to that, there was little central leadership for the church, and the individual communities were mainly interested in survival rather than finger-pointing and saying "You're wrong!" in loud voices.
Early Heresies
Marcionism
Marcion is famous for coming up with what was probably the first "Bible" - or list of canonical Christian works. However, it was a bit different to what we have today. Like most people who have actually read the Old Testament, Marcion found it a pretty disgusting work, and didn't find the God described therein a very good fit for the idea of an all-benevolent father figure. So, instead, he reasoned that the Old Testament was non-authoritative, that only Paul truly understood Christ's message, and that the God of the Old Testament was not really God, but rather a lesser "demiurge" figure that was responsible for all the evil and suffering in the world.
As you can see, it never really caught on, except for a few notable exceptions, which we will look at later.
Montanism
Montanism took off around the 2nd century AD, and mainly concerned itself with the idea of continuing revelation (ie, that there is still more of God's message to come), mainly based on a vague promise in the Gospel of John that humanity would be sent a "new paraclete". Notably, it also believed that once you sinned, that was it. You were out, and could not be redeemed. Pretty tough! Like modern Pentecostals, Montanists were big on the Holy Spirit, that part of the Trinity that to most other Christians seems to be a bit of a free-loading jerk.
Gnosticism
Gnosticism, as outlined by earlier Gnostics such as Valentinus, emphasised the importance of the "gnosis" (knowledge) to salvation. Gnostics believed that true salvation was only reachable once you obtained this ultra-secret super-awesome knowledge, and all the other plebs would have to make do with a lesser sort of heaven.
Gnosticism often overlapped with some ideas from Marcionism, and as can be seen from Valentinianism, there is a rather complicated theology developed here where, once again, the God of the Old Testament is the evil Demiurge, and the Christ is instead a being trying to link us with his true, spiritual, holy father.
Arianism
Arianism is one of the most historically significant "heresies". Based on the teachings of Arius, it's main contention with orthodox Christianity is that Arians believe 1) Jesus and God the Father are not of the same substance and 2) There was a time when Jesus did not exist.
Most Christians don't realise this, but the religions they belong to all accept that Jesus is eternally co-divine with God and, as the divine Logos, it was Jesus that created the world. Arians believed that Jesus, though Divine, was an inferior being, created by God.
As for the first point, this "substance" argument was quite heated back in the day. Once it was established that Jesus and God were divine, and the same being, then the question was asked, "Are they of the same substance?" Now, for many, the correct answer is, "Why does it matter?" but on the other hand, we're talking about people who were willing to debate the number of angels that could dance on a pin, remember.
This debate became summed up by the words describing the various camps:
Some of these were so similiar, particularly the first two, that many could hardly tell the difference. In fact, the first two, homoousios, homoiousios had nearly the same spelling, with only one Greek letter, "iota", being the difference. This led to sarcastic wit, whose name has unfortunately been lost to time, to quip that there was "not one iota of difference", a comment on the narrowness of orthodoxy.
Above: The source of all our trouble!
The Arians were condemned as heretics, and their teaching outlawed. Unfortunately most of the new Barbarian overlords of former Roman territory had converted to this form of Christianity, and the schism between Arianism and Orthodoxy led to a lot of mistrust and suspicion between the two groups, who allegedly shared a single faith. Only the Catholic Franks, who had converted directly to orthodox theology were spared from this, allowing them to integrate and mix their Frankish and Gallo-Roman populations far more quickly, giving them an edge in the era to come.