(This post will serve as an overview, and a follow-up post or two will look at particular cases.)
Religious extremism comes in two opposite degrees of adherence -- fundamentalism and abandonment (apostasy). By "extremist," we're talking about the zealous kind of fundamentalism, rather than mere traditionalism. And we're talking about a zealous kind of apostasy, rather than mere fading away or lapsing.
The resonant phrase "zeal of the convert" suggests that most fanatics about religion -- pro or anti -- have shallow roots in the religion in question, while those who are more level-headed about the religion must have deeper roots. This phrase refers to events over the lifespan of an individual, but it applies at a higher time scale to whole communities or cultures, based on when the religion was adopted by the group.
Those who are recently converted have not been participants in the developmental process that created the religion up to that point. So if the religion is nascent, it is hard to distinguish converts from originators in how much of a hand they've had in its development. However, if it has been around for awhile and has mostly congealed into a mature form, converts will be mostly passive consumers of an elaborate product made by a wholly different group.
So the religion will feel organic to the community that adopts it early on, while it cannot avoid feeling somewhat alien to those communities that adopt it much later on.
Indeed, to late adopters it has become so elaborate and so hardened -- allowing no further development -- that it requires a huge leap of faith to accept it, or else total rejection as though it were an organ transplant from a different species.
Even for those late adopters who accept it, they will question why there is such a long developmental process, from the origin of the religion through centuries of evolution. To the early adopters, all of those changes have been organic and internal -- solving the initial problems, or smoothing out the initial wrinkles, until we got it just right. But to the late adopters, that developmental process feels artificial, as though adulterating the purity of the original -- the ongoing profane work of man, not the completed divine work of the gods.
Thus late adopters tend not only toward greater zeal, but toward fundamentalism, or seeking to strip away the later encrustations to reveal the pure original. This leads not only to erasing all sorts of canonical beliefs, but also practices and rituals -- it leads to cosplaying as though you were a member of the original group that started the religion.
By taking a time machine back to the early days, these cosplayers can start their own traditions. The trouble is that every several generations, they will want to go back to the beginning all over again, and start another set of traditions. They therefore do not intend these as traditions to be kept by future adherents, but more like contemporary interpretations of the original -- to help it make sense to today's members, unmediated by centuries of actual traditions.
Strong adherents from an early adopter group, though, will appreciate the rich history and traditions that have put the flesh onto the original skeleton of the religion. For them, "going back to the origin" would be tearing off and discarding the flesh of the organism, just to gawk at its skeleton -- puzzling, and disturbing.
And at the other end of the adherence spectrum, late adopters who reject the religion are not just fading away or downplaying something they still kind-of believe in. They see the centuries of elaboration, that they played no role in, as proof that this religion is just a creation of man, and not a revelation sent from the gods.
This kind of apostasy is cynical, bitter, and dismissive -- not the kind from a lapsed member of an early adopter group, whose atheism is more trusting, bittersweet, and charitable toward the believers and practitioners. The late adopter apostate never felt truly part of the religious community, so there's no love lost for them.
The early adopter apostate did feel organically part of the group, and does feel a loss upon leaving them. They may say that they are "still culturally a member" while not a practicing or believing member. The late adopter apostate does not affiliate even "culturally" with the group.
Having reviewed the social psychology, in the next posts we'll look at some specific historical cases of this phenomenon. The most familiar place to start with is Christianity, contrasting those who were Christianized early vs. later. Then we'll look at the other world religion, Islam, and see a similar pattern.
Apart from religious concerns per se, this will also touch on foreign policy, as the LARP-ers in both religions are obsessed with the contemporary politics of the lands where their religions were founded. Given how influential these groups are within their home nations, their status as late adopters of their religion is crucial to understand their obsession with the current affairs of such distant and seemingly irrelevant lands.