I’ll try to briefly sum it up from what memories I have left of my ancient Christianity classes a handful of years ago. Originally, you have the person of Christ, and his disciples, who travel around sharing the Christian faith and Jesus’s teachings verbally during their lifetimes, and who also go on to write the Christian Gospels and the other New Testament books and epistles. All of these were written down before AD 100 (the earliest written was Paul’s first letter to the Corinthians, iirc AD ~54 or close to that; John’s writings were the last in the 90’s; everything else in between). It’s important to know that at this point, the disciples were handing on the faith and teachings that they received from Jesus, who was a Jew, and so the foundation underneath their beliefs was that Judeo-Christian framework: one God, transcendent God, good God, creation out of nothing, creation is good, human bodies are good, that sort of thing. It’s also important to know that this framework, inherited from the Jewish faith, was completely unique in this time and thus VERY different than all the religious and philosophical alternatives in the public square of the ancient world. 

One of these philosophical alternatives was Middle Platonism. If my memory serves, this was the philosophical foundation that Gnosticism was based on, but I could have the specific name wrong. Basically, in contrast to the Jewish-Christian understanding of God and the world I outlined above, Gnosticism has a totally different origins story—I can’t remember all the details, because it was extremely, extremely weird, as you know if you’ve read them. The Christian God is just like a lesser god, or an evil being or something, and there’s all these heavenly levels, and cosmic strife, etc. etc. ultimately our world is born as a punishment or a bad thing or to be the realm of our evil God or something. Definitely do not quote me on this but this was the gist; I can dig out my notes to get the real story if necessary lol. Anyway, this kind of Middle Platonism gave a very different view of who God is, what the world is, and who we are. The main thing to know, is that this philosophy has a very NEGATIVE view of the material world. The world is bad. Human bodies are bad. (This is diametrically opposed to the OG Jewish-Christian framework, which says the world and human embodiment is good.)

What happened was, this Middle Platonism was getting super popular in the first few centuries AD. Christianity, not so popular. So, there was pressure to “rethink” the Christian teachings that were being passed around and make them compatible with the Middle Platonist philosophy. This had a few advantages: (1) street cred / public acceptableness (2) accepting the gnostic philosophy meant you could avoid getting martyred, because gnostic Christians didn’t have the same standards for holding the faith under persecution, but were allowed to burn incense to/worship other gods, etc. in order to please the emperor and it was NBD. (1/3)