In my post Why the gods will never be defeated I made many references to the rise in religiosity concomitant with modernization in South Korean. Here is an article which illustrates what I'm talking about:
As recently as 1964, only a little over 3.5 million South Koreans, out of a total population of almost 28.2 million, noted a religious affiliation on government census forms. In other words, less than four decades ago, only a little more than 12% of the South Korean people declared themselves to be Buddhist, Catholic, Protestant, or a follower of one of Korea's many other organized religions. By 1983 more than 15.5 million South Koreans, close to 40% of a population of over 39.6 million, responded in the affirmative when their government asked them if they professed faith in any particular religion. That was more than a four-fold increase over the number of believers two decades earlier. By the 1990s, those willing to identify themselves as members of a specific religious community had risen to between 47 (in 1997) to 54 (in 1991) percent of the total population of South Korea. The size of the self-proclaimed religious population had risen from less than 16 million to between 21 to 23 million in a little more than a decade. Moreover, according to the 1997 Gallup poll, almost half of those who said they had no religious affiliation at that time confessed that they had once considered themselves Buddhists, Catholics, or Protestants....
Elsewhere you will note that the article suggests that religious affiliation is correlated both with urbanization & education. South Korea might very well be sui generis, the importance of Christianity is due both to the fact that during Japanese rule that religion was a vehicle of national resistance against colonial oppressors (who were generally Buddhist/Shinto), and, the close ties with the United States, a Christianity nation, after World War II (South Koreans also circumcise in imitation of American practices). The association of urbanization and education with religious affiliation, and in particular Christianity, seems reminiscent of the transition from godly city to pagan countryside during late antiquity. But with regards to South Korea is not the general explanatory power that is important to keep in mind, but the reminder it offers about the importance of particular & local contingent conditions. This is not to negate the power of macro forces, both historical & social, but to reiterate that sometimes the deviation around the central tendency is often only marginally less important than the modal phenomenon.
Note: Also, if you read the original article you will note that religious affiliation growth seems to have leveled off in the 1990s. This suggests that rates of growth are not constant as the innumerable parameters influencing are start to change dynamically. A simple explanation may simply be that only a subset (around 1/2) of South Koreans are naturally inclined toward religious affiliation in any case. The large number of "ex" members of various faiths and persistent between group conversion attests to the likelihood of meta-stability in this case.
this is a real mini-trend at elite universities insofar as evangelical christianity is becoming an 'asian thing.' nevertheless, the macro-dynamic is that asian americans have become considerably less christian within the last 15 years. see here.
The referenced article makes some mention of Christianity as a carrier (marker?) of Korean nationalism during the colonial period. It also mentions that many of the first modern institutions were created with missionary help. The article fails to mention AFAICT (other than maybe by implication) the importance of the missionary schools during the colonial period - basically the only way for Koreans to get any advanced education (as compared to e.g. Taiwan during the same period where the Japanese put a fair amount of emphasis on educating the native population; naturally the education was also indoctrination, and there wasn't the pre-1895 history with Taiwan that there was with Korea).
I also wonder if there was systematic bias in the reponses to the colonial period census the authors quote. How that response bias would play out I'm not sure, but presumably whatever a Korean at the time thought would cause him/her the least trouble. I also don't know if how one responded to the colonial census had any legal significance; I assume not (e.g. no difference in personal law).
It strikes me that this is another case of the kind of thing described by Geertz in "Islam Observed". In stage one you have the unchallenged tradition; stage two is the tradition challenged by an outside force (the West, or Japan), and local modernizers responding to the outside example, partly in imitation and partly in order to resist; and stage three, a return to the tradition in a reformed, fundamentalist form, which resists the outside influence while remaking the "corrupt" original tradition.
Traditional Korean religion was presumably like traditional Chinese religion: an eclectic mix of practices and beliefs put together by each family or individual for their own purposes from sources which included Confucianism, Buddhism, diffuse traditional local cults, charismatic movements, and possibly even Christianity. (Korea was more Buddhist and less Taoist than China, I think). In this context most people were vaguely religious without a clear affiliation, and "religious affiliates" were the minority who showed that were very serious, for example by dedicating their children to Buddhist monasteries or by converting to Christianity or a charismatic cult.
In this picture Christianity would function as the modernizing outsider. It's not too unusual for the progressive force to come in religious form -- in China missionaries taught modern science and modern medicine and were a connection to the global economy.
And in response, those who did not become Christian became more Buddhist in defense.
I don't know enough specifically about Korea to defend this sketch, but it fits with what I know generally about these processes elsewhere.
This reminds me of this:
...Gypsies, generally not known for their industriousness, are absorbing a work ethic via their new-found Christianity. I'm not religious at all, but Dawkins et al get on my nerves when they say that religion has no value. A set of values which captures the imagination can do wonders for some people.