would presume some people would wrongly attribute this phenomenon to something like the "Americanization" or "Westernization" of South Korea, associating Christianity to some kind of western or white prestige, but this is a very stereotypical and frankly racist view.

To understand the growth of Christianity in Korea, we need to understand the importance of Christianity as a counter-culture movement and not solely as a religious or spiritual experience. Of course, Christianity entered the Korean peninsula in the late 1800's as protestant missionaries (not Jesuits as in Japan). To me personally, speaking as a Korean Christian man, I very much admire the work they've done to develop Korea as a whole. These early missionaries did not achieve great societal and cultural penetration and Christianity did not really pick up its pace during this time, but these people contributed to building schools and hospitals, established newspapers in vernacular Korean (and not Hanja or chinese characters that was used by the yangban elites), and as a result, we can see them as promoters of Korean culture while at the same time proselytizers of foreign religions hostile to the Neo-Confucianism espoused by the Joseon dynasty at the time. The converts that did accept Christianity during this time were confined to the non-elites, especially in the less developed Northern regions (North Korea today) away from the capital where yangban influences were weaker.

The growth of Christianity really began under Japanese colonization, more so after 1910 when direct rule began, and among many other anti-Korean policies under Japanese authorities, Koreans were forced to practice Shintoism (the emperor as a divine figure) and build Shinto shrines on top of old Neo-Confucian ones. Of course, many people found this to be unacceptable, so Japan began to also support Buddhism as an alternative under the Soto sect (one of the schools of Japanese Zen Buddhism). In response to state sanctioned forced conversions of Koreans to Japanese style religions, many opt to convert to Christianity because many of the independence activists were Christian leaders as well. This changed the perception of Christianity in Korea as not a "Western religion" but a bona fide Korean religion which was also anti-Japanese.

In fact, the March 1st Movement (Manse Undong), the first public display of Korean independence movement was led by a majority of Christian leaders, who signed off the first Korean Declaration of Independence. They are the founding fathers (in a way) of the Republic of Korea.

So to conclude, the destruction of Korean cultural traditions by the Japanese led to independence movement leaders to seek new ways of opposing Japanese policies. Christianity presented a new way to do so. To add: Christianity was a religion of national salvation.

I will finish by adding that it's certainly true that there was a degree of "Americanization" in Korea after the liberation. That of course was NOT the promotion of Christianity, rather the opposite. American leaders pressured Rhee Syngman to adopt religious freedom for Koreans. Rhee, who was converted to Christianity while imprisoned by the Japanese, wanted Christianity to become the dominant religion, a true religion that liberated Korea and one that would protect the new Republic of Korea against communism. Early on, there was a soft policy that made Christianity the state religion until he was deposed. The trend has been for some time that the percentage of population that identify as Christian has been decreasing, and the definition of what it actually means to be Christian is becoming looser (like do you go to church every sunday? etc.)

Source if you're interested in reading:

Djun Kil Kim A History of Korea (A general overview with a pro-Korean slant especially in ancient history)

대한민국 건국과 기독교 (edited by 박명수, 안교성, 김권정) sorry for the Korean source, the title translates roughly to Republic of Korea establishment and Christianity

Hope this helps

/u/APRK has already answered the question but I would like to add few points

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Japan and Korea have a very different history and culture, which is why Christian missions have produced very different results.

In Japan, almost five centuries after the arrival of missionaries with Francis Xavier in 1549, those baptized into the Catholic Church number 440,000 out of 128 million Japanese, 0.35 percent, while the Protestants number around half a million.

In Korea, where Catholicism arrived with a few laymen at the end of the 18th century, there are approximately 5.3 million Catholics out of 50 million inhabitants, more than 10 percent, while the Protestants of various denominations add up to about 8 million, 17 percent. At night Seoul looks like a Christian city because of the great number of crosses on churches, schools, hospitals.

The Christian faith has been received with great difficulty by Japan, while on the contrary South Korea seems to receive it with open arms today. From the 1970's until today about half the presidents of South Korea have been Christian.

Why do the Japanese convert in such small numbers?

The religions of Japan, like Shintoism, teach that man is one of the many elements of nature, in which the unknown God is manifested.

Confucianism presents a static vision of society, where the supreme moral norm is respect and obedience for the sake of maintaining harmony between heaven and earth, between superiors and subjects, between politics and economy. According to Confucian morality, each one must carry out his work with the greatest diligence in the place that has been assigned to him.

Buddhism, teaching detachment from oneself, disdain for the passions and personal ideals, which are considered harmful illusions, makes the individual willing to accept anything and patient beyond measure.

The Japanese is a child of these religions: an excellent worker, sober, obedient to commands. In a society where everything must function like a machine, the Japanese is the ideal subject, because he acts as part of the group. The people have a powerful awareness of their unity, but little awareness of personal rights. Common life begins in the family, continues in the schools, and ends in the business firm, which is conceived of as a big family. The spirit of collaboration that predominates in the company makes work highly efficient and productive. The success of the firm for which one works is considered an ideal of life worthy of self-sacrifice, even with hours of overtime that are often repaid with little or nothing

South Korea is radically different. Over the last half-century it has seen a record growth of Christians.

Most of the conversions took place in the cities and among the country's elites, professionals, students, artists, politicians, and members of the military, including some at the highest levels. The symbolic representative of the Catholic Church in Korea has been Cardinal Kim Sou-hwang (1922 -2009), archbishop of Seoul from 1968 to 1998, the proponent of strong Catholic outreach in the social sphere. During the long military dictatorship he made the Myong-dong cathedral in Seoul a refuge for the nonviolent opponents of the dictatorship. The military never dared to enter the cathedral, which they knew would be defended by the people. For many years Cardinal Kim was the most influential personality of Korea.

Korea also saw half a century of Japanese occupation and then more than three years of civil war between North and South (1950-1953)

Sources: Everlasting Flower by Keith Pratt

The History of Korea by Djin Kill Kim(hope the spelling is right)

A Concise History of Modern Korea by Micheal Seth

These are just the top off my head