Probably the most difficult aspect of studying the origins of Christianity is that there is so much history to plow through. If you pick just one place, you can spend lifetimes exploring the peoples and families that had just been there over the centuries.
We know something about the peoples who lived in Mesopotamia in the ancient past because those people wrote some things down. To some degree, we can witness the ancient tribal customs evolve into urban traditions. We can discern the effect of the early Greek cultures encountering the Aramaic and Egyptian and Persian cultures.
Like an early Arctic explorer who discovered to their dismay that the closer they got to their goal, the more useless their compass became to them, I'm finding that the closer I get to the origins of Christianity, the less I seem to be able to grasp.
We know that the great early centers of Christianity at the time of Constantine were in Alexandria, Antioch, and Rome. What's difficult to pin down is how, exactly, these areas became such hotbeds for this new faith and not others. The "official" list of "Popes" of Alexandria starts with "Mark the Evangelist" -- y'know, the guy what wrote the book of "Mark" based on all this secondhand knowledge he gained from Paul. And this sounds convincing, except for the fact that the earliest histories mention another guy, Anianus, as being the first bishop of Alexandria, with some suggesting that Mark himself anointed the guy on his way through town because Anianus happened to be the cobbler that fixed Mark's shoes. That, and it's unlikely that the book of "Mark" was even written until a few years after "Mark the Evangelist" was dragged to death by horses at the behest of some aggravated townspeople.
Okay, then: Antioch. The first bishop was Peter the Apostle on all the official lists, with the story being that he went on to Rome not long after. But not everyone agrees that Peter had anything to do with Antioch. The first bishop everyone agrees on is Evodius, who was ordained Bishop in Antioch in 43AD and is credited with the first use of the term "Christian". What's more -- he wasn't a martyr: he died of natural causes in 69AD. It was his successor, Ignatius of Antioch, who so burned to become a martyr that, after 30 years of making trouble, did eventually end up in an arena in Rome. So who anointed Evodius? We're told it was Peter -- y'know, on his way to Rome.
Setting aside the argument that Mark and Peter wrote whatever they were supposed to have written, Ignatius of Antioch is actually one of the earliest "Apostolic Fathers" for whom we have several letters presumed to be authentic. The problem is that he is an exception to the rule that all of the bishops listed for the seats in Antioch, Alexandria, Jerusalem, and Rome are equally mired in obscurity. Most have no references for them beyond their presence in the list. Importantly, we have no record of any contributions each may have made to the body of theology and practice. There is enough early controversy regarding these lists that it's clear they cannot be taken at face value, but is there ever a point where they are to be believed?
I can point to Alexandria as the source of the Isis-Serapis cult, and Antioch as the origin of the Baal cult, because we have Roman sources that record when these things arrived in Rome and where they came from. We have letters from soldiers initiated in Antioch and Alexandria, so we're pretty sure we know what was going on. Fast forward a few hundred years and we see significant theological differences between Christians in these two locations and I wonder if there is a connection. Had Evodius been the one who formed a spin-off of Neo-Platonic Baalism to Christianity? Did Anianus reformulate the Serapis cult to recapture a Roman audience? Most importantly, what was the connection between these people that we would understand today?