The following text outlines my current understanding regarding the origins of Christianity. As usual, it's chock full of speculation, which I like to pretend is rather well informed speculation, naturally.
What it is and isn't
Christianity has deep roots within Judaism, but it is not an Aramaic faith. Greek philosophy informs and structures the theology and politics of early Christianity, joining together elements of Egyptian, Persian, and Phrygian cults, yet it is much more (and much less) than any of these individually. Part of the reason we have such difficulty understanding these origins is that they lie behind a period of unprecedented cultural reach and multi-national trade unrivaled until the 18th century. This period of global prosperity was followed by a thousand years of bad weather, bad economies, and bad politics which further shaped the self-image of the Church.
However, despite the accounts of the Gospels, there is precious little evidence of a single, guiding hand bringing together disparate elements of Universal Faith into a Sacred Whole. Instead, the best explanation for the origins of Christianity depends upon an organic process of trade and debate, conquest and resistance, to create the material evidence of culture that we find today. There were many men who made specific choices that resulted in the creation of the Christian Church. But these men lived over many centuries and rarely worked consciously to create a universal religion -- that is, until the mid- to late-second century.
Savior Guisada
The particular stew that eventually made the big Christian taco arguably dates back to Egyptian dynastic struggles. At its political peak, Egypt dominated the Nile and the Jordan, with significant trade between Memphis and Byblos connecting the Mesopotamian empires to the Pharaoh. Even after losing political dominance in the area around the Jordan, Egyptian culture continued to be honored, leading it to form political alliances it was ultimately unable (or unwilling) to support.
Aramaean culture dominated Mesopotamia for a thousand years. Assyrian and Babylonian (Chaldean) cultures both derive from Aramaean roots, as do Jewish, Samaritan, and Phoenician (Carthaginian) cultures. There was consequently a great deal of trade of both material and cultural goods between these peoples. The "exile" of the Judean princes to Babylon was less imprisonment in a far-off, difficult and exotic locale than an extended visit with cousins in a legendary urban area of luxury and power -- like moving from Waco to Dallas.
Greek culture, on the other hand, was quite novel in comparison. The intersections of Greek cultures are writ large in the writings of the peoples of the western Mediterranean coast. Although initially hostile, Western Mesopotamian cultures eventually warmed up to the Greeks, blending into a thick, frothy paste. The action arm behind this power blending action was driven by the armies of the Egyptians, Assyrians, Babylonians, and Persians. After waves of these left established cultures around the Levant reeling, both groups found the best in each other to lean upon.
For two thousand years, armies marched, dynastic struggles and civil wars raged. Each left a demarcation on the psyche of the peoples in the Levant such that every area was balkanized into a multitude of variously warring tribes. The Persians and the Greeks maintained a policy of keeping "friendly" populations politically intact, both increasing their economic value and improving the morale of the citizens. In practice, this acted to cement the divisions between the various tribes and nations by recognizing them as official boundaries, even when the cultures of those involved were essentially identical.
Judean Struggles
One result of this division is seen between the Jews of the Temple and Tent. Many of the Aramaic tribes found it easier to relocate than to remain near to Jerusalem between the spears of conflicting armies. This first diaspora into Greek lands laid the social groundwork for the many successive movements. Those who stayed near to the temples in Jerusalem and Samaria maintained their traditional practices. Those who went far away adopted Greek ideas, Greek philosophies, and Greek practices. Projecting into the future from this point, Temple-based Judaism was destroyed by the Romans, but these Helenized "Tent" forms eventually became Synagogue-based Judaism.
The arrival of the Persians formed a third faction by "restoring" the Judean princes to political and ecclesiastic power, alienating the previous Temple hierarchy and perhaps further disenfranchising the people. While the comings and goings of the Greeks probably did little to ameliorate these divisions, the very act of breaking free of Greek power created yet another division between those who were in power and those who gained power. The Jews had very nearly convinced themselves of their own self-sovereignty, until internal struggles invited the Romans through the front door.
It was during the power struggles against the Greeks and the Maccabees that utopian, fundamentalist movements were created, to which early Christianity can trace its earliest forebears, such as the Essenes and Gnostics. By the arrival of the Romans, these groups had advanced to hero worship of their founders, adopting techniques of the Greek philosophical schools to structure and promote their ideas. That these are the sources of later Christian groups is shown in how the words of the "Teacher of Righteousness", the anonymous leader of a fundamentalist Yahwist group tentatively dated to around 100BC, are found later verbatim in the mouth of Jesus of the Gospels.
Early Fundamentalism
The "Teacher of Righteousness", about whom we learn from the Dead Sea Scrolls, was vehemently opposed to the Maccabeean leadership in the Jerusalem temple to the extent that he led an offshoot group claiming to teach and live by a more pure form of Judaism than that of the corrupted Temple leadership. (Maybe these are the Essenes?) That story about upending the marketplace near the Temple makes far more sense had the actor been a disenfranchised member of the leadership elite of that city, like the "Teacher of Righteousness", and not just another visiting "Rabbi Josh" from the sticks. There may also be a connection between the group formed by this "Teacher of Righteousness" and the cult of John the Baptist.
Besides the likelihood that it really was something of a stretch for even the Roman Empire to hold the Western Med coast, I suspect that the Romans found exceedingly unpleasant echoes of the hated Carthaginian culture within related Aramaic cultures, and because of this, probably treated the Jews a little more harshly, and with a greater expectation of treachery, than they might normally have. Yet the Jews were peoples hardened on the anvil of generations of war between the greatest peoples of the age, and so were likely less willing to be cowed by the mere display of military resources. In any case, the relationship between the Romans and the Jews (inevitably?) soured.
The Jews were responsible for not one or two, but five major insurgencies, requiring the presence of anywhere from four to six standing Roman legions in the area to keep the peace. Eventually, the Temple in Jerusalem was razed, and Jerusalem itself was refounded as a Roman city. Many of the Jews that had lived there moved to cities in Persia and to the Greek cities that had established Jewish populations. This wide distribution of disaffected, disenfranchised, and socially well-organized culture resulted in a Graeco-Syrian economic underground throughout the Roman Empire that relished its cultural heroes and violently fought against homogenization into the Roman culture.
Greek Contributions
Greek philosophy had, by the time of Roman occupation, developed to the point of rationalizing all religion as sacred components of a perfect faith devoted to a single, perfect deity. How this monotheism was expressed differed from one place to the next, but the Greek genius was to recognize all of these manifestations as being "of the same essence". From a very early point, Greek philosophers recognized and respected the monotheistic emphasis of Judaism, viewing this faith as an early proponent of truth as they understood it. Consequently, there was a great deal of crossover between Jewish and Greek cultures after the imposition of the Roman Empire.
Another symptom of this universal synchretism was the adaptation of the faiths of those around them, if only to come to a closer understanding of the perfect worship of the perfect deity. It was in this way that the ecstatic musicology of Phrygian faiths joined with the solemn reincarnation of the Egyptians and the Mysteries of Dionysos and Eleusis to form a pastiche of shared sacred experiences among the urbanites of the Roman Empire. Counted among these experiences for many was that of Jewish Yah worship.
These Greek cults formed themselves along the same lines as the Neo-Platonic philosophical schools. Early Christian groups were no exception, with each particular arrangement of Greek, Asian, and Egyptian elements described as the teachings from the founder of each school. The need to politically connect these theologically similar groups led to the presumption that the founders themselves were followers of an earlier teacher, thus demoting the established leaders of their schools to 'Apostles' of a previously little-known figure, but making the larger body of 'Catholic Christianity' stronger. At the same time, the intense swirl of Persian, Syrian, Anatolian, and Egyptian faiths in the genericizing vat of Graeco-Roman culture meant that "Christian" groups were exceedingly diverse and derivative of nearly everything that came before.
The Conversion Process
Some Greeks fully converted to Judaism (walking away from their family and city gods, getting circumsized, and adopting new diets), while others hung around for the social networking without putting up with all of the taboo and glad-handing. Consequently, around every Jewish community in every Greek city orbited some number of gentile Greeks in various stages of conversion. They shared much of the Jewish culture without the limitations or obligations regarding its maintenance. Although it was among the Jews that Christianity first spread, it was from these gentile "groupies" that it spread radially outward through the Empire to became a force to eventually dominate it.
These early gentile Christian groups had no hesitation regarding including all of their religious experiences and languages into their new passion. The rather predictable result was that the weft of Cybelene, Isiaic, Mithraic, and Neo-Platonic ritual, theology, and practice were woven through the warp of Graeco-Syriac culture -- in some cases to the point of overriding the underlying Asian elements. Narrative stories regarding the "founder" of the Christian cult had been a long-standing part of the cult mythology, but it wasn't until they began to be written down around the beginning of the second century (AD) that a process of regularization and synchronization brought the disparate sources together into the four recognized Gospels we have today.
The "golden age" of early Christian development runs between about 100 and 325AD, during which time, various theological and philosophical truths were sculpted by the more inventive and opportunistic bishops. After the appropriation of Christianity by the Roman state, many of these truths lost their relevancy and remained forgotten until the Reformation. Although Constantine was a significant force in the merging of Christianity and the Roman state, his grasp of theological concepts was shallow at best, and in any case came along far too late to have a significant impact any of the debates of his day, other than to end all arguments as they stood.
Constantine provided to Christianity a strong need for regularization and an enforcement process by which to maintain it. By these means did the story of Christ, the theology, and the corps of ritual resolve to the features we would more recognize today. Many of the myriad forms of Savior worship common before Constantine would be strangely familiar to us today, although not always in contexts we would recognize. It was Constantine who demanded uniformity and who was largely responsible for the shape of the Church for the next 1000 years. As such, I currently date the beginning of Christianity (as we know it) to the Council of Nicea in 325AD. All that came before this were simply the fruits of the free, international trade of ideas and culture in the Greek mode, provided by Alexander's conquests.