Speaking from a vantage point that does not care
ecclesiologically about Europe and its dumb, collapsing empire full of idiots (and some saints, yes
), all explanations so far given are wanting in certain ways. To deny that there was any politicking involved just seems silly. You need only read books like Adam Schor's
Theodoret's People: Social Networks and Religious Conflict in Late Roman Syria (University of California Press 2011), or even the relevant portions of
The Dictator's Handbook: Why Bad Behavior is Almost Always Good Politics by Bruce Bueno de Mesquita and Alastair Smith (Public Affairs 2011) to see that there was plenty of political networking and alliance-building going on, in both the Western and Eastern Roman empires. There's no having an empire without it, no matter what religion that empire professes.
But the further you get from the Western centers of power and their reasoning for doing this or that (e.g., who goes on the calendar when and why), the less all that tends to matter. Christianity was also established from a very early date in empires completely outside of Rome or Byzantium, as at Axum (c. 330) and in India (52 AD), and of course also in the neighboring (and hostile) Persian empire. All of these were through contacts established with the West, sure (as Jerusalem and Alexandria and other early centers of Christianity were in the Byzantine Empire, which was largely west of the places I'm talking about), but then these were also all evangelized before the establishment of Christianity as the religion of any empire -- in some cases, long before. So the reasoning that Constantine or whoever might've seen political advantage in adopting it or sanctioning it, while it may make a great deal of sense in a Roman Imperial context, doesn't really say anything regarding how sensible it would've been to be adopted by Persians, Syrians and Armenians in Mesopotamia, Axumites, Keralites, and so on. And relative to these same people, the bulk of Europeans were rather late to the party. Christianity spread through North Africa and nearby parts of Mediterranean Europe very quickly, but seems to have had trouble moving northward or eastward, as the comparatively late dates of the Christianization of Hungary, the Slavs, and especially the Balts shows. Even a place like Britain, though it had been producing saints for centuries by that point (the third-century martyr St. Alban comes to mind), cannot really be said to have been "Christianized" before the first king to accept baptism, Æthelberht of Kent, who was not baptized until c. 601 AD.
So it really didn't spread that fast at all, when you think about it. It arrived very early in certain places, but then it generally took a while. Even the people who we think of as most solidly Christian, like the Syriacs, took until about the fourth century before they were more or less completely Christianized. Same with the Copts, though it's hard to date this kind of thing because we can't really take something like the closure of the pagan temples in the 380s (on the order of the Byzantine emperor) as a sign that there just weren't any more pagans. We have dated examples of graffiti written in Demotic that long post-date that time (Coptic Christians did not use that writing system), proving that the Egyptian pagan community must've survived at a very low level for some time after that.
Anyway...most people don't have these places on the edges or outside of the Greco-Roman empire in mind when they talk about Christianity, but since I resemble these "back country and rural village" people, I do. Europe is nice and all I guess, but if it had never been Christianized, chances are plenty of other people still would have been.
As far as what it has to say about humanity and all that, eh...that should be pretty obvious if you've ever read the NT, been to a Christian liturgy, etc. I don't think there's any one message or style of presenting that message that could explain its spread, because obviously a great deal depends on what appeals to particular people and how they interpret it and flesh it out within their own context (read: it's not a coincidence that Calvinism did not come out of the Egyptian monasteries or whatever, nor Sola Scriptura out of the School of Nisibis, etc.), but there is a lot to be said about the places that the early disciples went, and who they specifically preached to, and how things went from there at particular locations. Like it was probably pretty smart that St. Mark, who was a Hellenized Libyan Jew, went to Alexandria -- a major city with a preexisting Hellenized Jewish community that was obviously dedicated to the scriptures (the LXX having come out of that community in the time before Christ) -- whereas other people went to other places. I think a great deal of the subsequent
failure of Christianity in spreading to or recapturing (depending on how you look at it) certain areas came precisely because of this evolution away from understanding the situation on the ground, as when you had hordes of Western Crusaders coming into Jerusalem and not only doing horrible things to the Muslims there, but also eventually trampling over the preexisting Christians in an effort to establish kingdoms and dioceses with no respect or understanding of the prevailing order. Not only does it call into question the true motives of such people, it's really deaf to the way things are in a given place. And this kind of thing seems to have carried on in the age of European colonialism, only loosening in the 1960s or so in the Roman Catholic case, with various attempts to "nativize" the standard Latin liturgy, e.g., the Misa Bantu, Misa Folklorico, etc. experimentations. Some of these, I must say, are quite aesthetically pleasing:
Though it's almost certainly not anything like what would've been found in the indigenous African Orthodox churches of Christian Nubia (as those were in communion with either the OO or EO, depending on which kingdom you're looking at), at least they're trying. If this connects with Senegalese people, why not have it.
Not sure if that answers any of the OP's questions or not. Why did it spread as successfully as it did? Networks, eventually the backing of a very large empire, and various individual or collective missions to places outside of the empire's direct reach. But it wasn't as quick a phenomenon as, say, the rise of Islam. And so it didn't so much sweep everything away as nativize or baptize (whatever you want to call it) what could be brought in, and leave out what could not. And eventually it stopped being so successful, both due to competing religions and its own association with Western powers who don't know any other ways of being Christian and stuck patterns of being emperors and kings and all that other stuff that really have no relevance to a great number of people around the globe anymore, or even worse was resented on account of its colonialist connections.
(In the same way it is often turned to by those who are sick of being colonized by other religions, just by the way; this has been, for a few decades at least, the case with regard to some of the indigenous non-Arab people in North Africa, for instance, where some places in Kabylie in Algeria are approaching 5% Christian, which doesn't seem like much until you consider that this is all home-grown activity in a country that not only does not provide support for Christianity, but actively seeks to curb it by shutting down the mostly-Protestant home churches and such as illegal assemblies, which I suppose they technically are. Much closer to home, I have met Hispanic people and various sub-Saharan Africans in the Coptic Orthodox Church who came to it after being Roman Catholic and expressed some variation of "I don't want to worship in a church just because it was what the Europeans who colonized my country wanted my grandparents to do when they colonized us." Can't argue with that. I wouldn't want to either, and Islam is terrible all around.)
