Back to the OP, I actually half agree with Stephen that Christianity had a monastic origin. Further I agree that the Marcionites were not the starting point, but one of several sects at the moment evangelist erupted. In fact diversity was so great that we cannot trace the origins back to any single sect. The moment the first Christian literature of what would become the NT became public diversity of sects was already a defining characteristic of Christianity. I arrive at that, not through church father analysis, which I consider suspect but rather from scripture itself, Sola Scriptura (a nod to my Protestant heritage there).
Digression a bit. The concept of early or even pre-Christian "Jesus Communities" is not new and not considered a particularly radical concept. This arose mostly out of Q studies, and Kloppenborg-Verbin gives a rather detailed summary of the Q studies view of this in his 4th chapter Excavating Q (pages 166-213 in the paperback edition I have). This concept came about to explain the diversity of opinion in the very earliest Christians which is reflected in the NT texts and the battles over the "true" way. It's the problem of sectarianism from the very start of the written record, although many traditional or even partly traditional scholars still adhere to some form of the "virgin" church myth. Back to Kloppenborg-Verin's book, the first thing covered is the illiteracy rate in the Mediterranean, such that at most 10% of the population could read, if that. And how the earliest Christian texts, inheriting from the early LXX manuscripts such as P266,
https://upload.wikimedia.org/wikipedia/ ... ap_266.jpg, used scripta continua and required almost a performance type reading.
At this point my view diverges greatly from the views presented of a Jewish, perhaps Jerusalem based sect, that was within Judaism, and specifically Palestinian Judaism. This view I think is very wrong. I also think the view presented of these communities, were primarily peasant based. I am more inclined to view these Christian or "pre-Christian" communities as monastic (with Huller) and outside of Palestine (with Detering -- I diverge from Detering in his view that Indian Buddhism was the primary influence, as I believe we may be looking at something more akin to convergent development rather than a direct dependence).
There were communities such as the Theraputae which Philo details that fit the bill. I am not saying there is a direct relationship for sure, but their monastic communities far away from Palestinian Judaism, practices the sort of intense exegesis and performance plays and communal readings where an intensely unique divergence from mainstream Judaism could easily begin. But more importantly, these communities spread throughout the Greek speaking regions of the Mediterranean and seem to have been well along the way of elevating Joshua above Moses when Philo wrote about them. I doubt this Alexandria region community Philo wrote about was unique, as there were probably other more Christian like sects of this movement in other locals such as in what today are Greece, Turkey and Syria. There very spread out nature, semi-isolated from the mainstream society, each with it's own strong leader, were ripe to split into differing readings of the same texts and differing theologies. Such an incubator would have been perfect for pre-evangelizing Christianity of Jesus/Joshua followers. The fact that they were in Greek speaking areas and had been separated from mainstream Judaism for at least a century, suggests their membership was very likely not very Jewish at all, and I would suggest nearly completely Greek ethnic by the time evangelism erupted.
This eruption I place in the 2nd century. The one thing I credit the Marcionites with is the Gospel and evangelism to those not committed to the Monastic life. Their success in this initial evangelism, and their Gospel, is what caused a reaction from the other monasteries, as their theology was at odds with many of them -- the diversity was already pronounced the first day evangelism started. The reaction led to one book after another being written or revised in rapid succession as each group wanted to adjust the message to something closer to what their Sect (monastery) was preaching. This is where the legend of the Petrine counter mission came from. It is the friction in the story telling (of the legends) repeated in Galatians.
While all this is speculative, the support for this goes back to the scripture itself. It is long been noted that the stories in the Gospels are largely retelling of OT stories and themes in a new setting with Jesus, and with new message or theology which is synthesized with many non-Jewish elements. This pre-Gospel where the stories accumulated, must have come from the type of intense study one sees in a monastic or university setting. (A more recent example can be drawn from the Protestant Reformation and how quickly it spread out of intense internal settings in a small German speaking area to engulf most of Europe in less than two generations. The diversity in Protestantism was present the day it erupted as it had been incubating for a century or more before erupting.) So then the Marcionite Gospel is but one version of this internal monastic document, common yet variant from one sects monastery to another, which was built upon and honed for their evangelism. Matthew, John and Mark were similarly crafted as response, Matthew and Mark specifically from the internal document, John and Luke derivative (and of course others probably lost).
It was competition due to the diversity before the eruption that forged the NT and it's internal conflicts. All that could only have come about in an intense internal environment, and the monastic setting seems to fit that best. For once I think Stephen Huller has it right, although he probably has very different circumstances.