To anticipate certain answers:
The lack of Christians in Japan would have to do with the repression and forced conversions of Christians by the Japanese government during the 16th century.
Korea also banned Christianity in the 18th century--although I can't speak to the relative extent of the persecution.
As well as the the historical lack of devotion to one particular faith, indicated by the mixture of the Shintoism and Buddhism...
Korea also traditionally believed in a blend of religions and philosophies including Buddhism, Taoism, Confucianism, and indigenous shamanistic beliefs.
American influence in Christianity in East Asia is negligible.
Christianity has been in Korea since 1603 when a diplomat brought Catholic texts back from China where he received them from a missionary. During the 17th and 18th centuries a "handful of Koreans on diplomatic missions to China met with Western missionaries..."^1 so the seeds of Christianity had been planted 342 years before the US kicked out the Japanese. Christianity didn't take hold right away however as "most Koreans were highly dismissive of Christianity for many of the same reasons they objected to Buddhism: it promoted selfishness, honored celibacy, and gave credence to miracles."^2 This didn't stop some people from becoming Christian near the end of the 18th century, most notably the Namin faction of the yanban (upper class) who were out of power and rebellious. Numerous intellectuals converted and continued to spread the word.
It was during the mid 18^th to mid 19^th century that persecution of Christians began. In 1758 when Catholicism was officially banned by King Yeongjo, and in 1785 it was declared heresy by King Chongjo who banned the importation of all books from Beijing in 1786 for fear of Catholic doctrine being among the books. In 1791, a yangban son was sentenced to death for failure to honor his ancestors. This was tied to a ruling from the pope in 1742 that declared ancestor worship (a key tenet of Confucianism and East Asian culture) was incompatible with Christian ideology. The persecution was relaxed during the 1790s when a Namin yangban held some influence over the king in his last years. A priest from China came to Korea and there were an estimated 4,000 Catholics on the peninsula. But the king died, the dowager queen took over and the Catholic Persecution of 1801 took place.
It should be noted at this point that Catholicism was tied to class and factions. The Namin clan, out of power, had taken to Catholicism, while the Queen, from the Gyeongju Kim clan, had not. She persecuted the Catholics as a way to persecute her political enemies and, it turns out, for good reason. A Catholic convert, Hwang Sa-yong, sent a letter to the bishop in Beijing asking for the pope to get the Chinese emperor to ensure religious freedom in Korea and to have the European powers send a naval force to force the Korean government to do so. This played right into the governments hands that Catholicism was dangerous and during the Persecution, over 300 Catholics were executed. Over the 19^th century persecution would intensify and relax depending on who was in power. Notable instances were the Persecution of 1839 where 75 converts and 3 foreign priests were killed. The French sent a naval detachment in 1842 to investigate killings. In March 1866, 9 French priests were killed and in October 1866 the French responded with 7 ships and 600 men who captured a fort on Ganghwa island. They left a few months later when Korean troops pressured them and the onset of winter. But by 1864, there were approximately 20,000 Christians in Korea.
Christianity continued to grow with missionaries arriving in Korea. Ehwa Womans University was founded by the American Episcopal Methodist Church and Severance Hospital/Yonsei University was founded by the Presbyterians. (I'm personally fond of Severance as my aunt was a missionary there and was responsible for the first heart-lung machine in Korea.) Korea University was founded by Koreans who were associated with a religious order.
Another interesting point to note is that a big center for Christianity in Korea was Pyongyang. It was referred to as the "Jerusalem of the East" in the early 20th century. The Presbyterians especially had a foothold there and Kim Il-sungs grandfather was known to be a minister and his father was an elder in the church. Communism and atheism, Christianity and ancestor worship - the Kim family has some pretty crazy logic to get around all those holes in their story! But I digress...
We can see that Christianity was well established by the 20^th century with very little American influence. How did it flourish after the war?
Labor politics.
After the Korean War, South Korea was bent on economic growth and industrial output often at the expense of the people working there. Various Christian groups had tried to gain converts among the working class and started proselytizing to them. A Methodist/Presbyterian group, the Urban Industrial Mission, originally tried to gain converts but became increasingly concerned about the working conditions. They would hold bible studies that devolved into discussions on the factory conditions. A Catholic group, the Young Christian Workers, was established in 1958 and followed a similar pattern. Using their international connections, both groups brought in foreign speakers which made it difficult for the Korean government to crack down on them.
There were leaders that came out of the Christian movements as well. Cardinal Kim Su-hwan of the Catholic church is the most notable. Park Chung-hee once complained of the three Kims that were causing him trouble - Kim Dae-jung (a Catholic whom he almost had murdered), Kim Young-sam, and Kim Su-hwan. Kim Su-hwan met with Park on a few occasions and tried to get him to moderate his stances and it was his position as a cardinal and the strength of the church that backed him up, both things that would have gotten him murdered a hundred years earlier.
For more information, check out Christianity in Korea by Robert E. Buswell and Timothy S. Lee or Christianity in Modern Korea by Donald N. Clark. Buswell and Lee is only about 12 years old, so it's not too hard to find, but the Clark book is 32 years old and a bit tough. For a general overview (and the two quotes I sourced), Michael Seth's A History of Korea has a basic rundown and A New History of Korea By Kibaek Lee has a good section on the 18^th and 19^th century.
Part 1
Christianity has a pretty long history in Korea. A brief summary is that catholicism came first in the 18th century and experienced a series of repressions. Independent of that protestant missionaries came in the 19th century and took root when american missionaries gained support from the monarchy. Both survive to this day and are in practice regarded as separate religions (as opposed to being branches of the same religion).
Early Catholicism
The first ever contact between koreans and with any type of christianity was during the japanese invasion of 1592-1598 (Imjin Waeran). A portuguese priest came to korea together with the japanese forces, though he had no free contact with koreans while there, and some of the captives taken back to japan were converted to catholicism. Within Korea itself though that first contact had no permanent influence.
Of more permanent influence was the Jesuit presence in the Ming court, specifically their translations of religious texts into classical chinese script. Those texts found their way into Korea over time and attracted interest from some scholars, initially as a topic of philosophical (as opposed to religious) study and subsequently as a religion. One of those scholars, Seung-Hun Yi, was eventually baptised in Beijing in 1784 and became the first catholic convert in Korea. On his return Yi baptised two other scholars and this is generally seen as the formal start of catholicism in korea. There were approximately 4,000 converts by the time the first non-korean catholic, a chinese priest by the name of Mun-Mo Jo, entered Korea in 1794.
Disapproval from the confucian establishment followed due to the refusal of catholics to follow confucian rites such as those honouring ancestors because they were deemed to be “ancestor worship” and therefore heretical.
There were a number of suppressions over the years but the most noteworthy one took place in 1866 when 8,000 (out of 23,000 total converts) were executed. Nine french missionaries were also executed and this caused conflict with the french. The french temporarily occupied Ganghwa island, west of the Han river estuary and the same island that the americans would attack in 1871, until forced to leave by a korean counter attack. A bit of trivia is that the french raided the literary stores on the island and took nearly 300 volumes of the Uigwe, a book collection describing Joseon court protocol, which they returned in 2011.
World politics, and particularly the intrusion of western powers into north east asia, needs to be taken into account when looking into the reason for these suppressions. Western missionaries and their converts were seen as a threat because they were often used by foreign powers to intervene in countries where they existed and indeed the french occupation of Ganghwa did nothing to dissuade the court from this belief or diminish its resolve to resist such foreign assaults. The experiences with the americans in 1871 and the japanese in 1875 made it clear that simple resistance would not be sustainable, leading to the change in policy in 1876 that enabled foreigners access to the country and as a result ended the persecution of catholics.
A bit of trivia is that korea is said to have the 4th largest number of catholic saints and is the only country where catholicism emerged without foreign intervention.
From the 1880’s onwards to 1950, the catholic church in korea was generally quiet and kept to itself. It did not involve itself in social issues as much as the protestants did for instance. This is partly due to the church hierarchy transitioning from being led by korean laymen to a clerical structure of mostly foreign missionaries who favoured a separation of the religious and the secular.
Early Protestantism
European missionaries were the first to make contact with koreans. One of the earliest known is the german Karl Gützlaff, who was on a merchant ship visiting korea in 1832. Generally speaking though protestantism did not take hold until the opening of the country in 1876.
One of the more significant milestones in this process was the translation of the new testament into vernacular korean. Prominent in this were scotts John Ross and John McIntyre who with the help of converts translated the bible into korean between 1882 and 1887. Yi Su-Jeong similarly undertook a translation in 1885 while in Japan. These translated texts were distributed by colporteurs and most koreans would’ve met the religion via text before they came into contact with missionaries, mirroring the manner in which the deep literary tradition of the country helped the spread catholicism in its early days.
American missionaries are widely noted as being influential in the spread of protestantism in korea. Key among these were the medical missionaries who befriended the court, key among them being Horace Allen. At the courts behest Allen initiated the first modern medical hospital in Korea in 1885, the Gwanghyewon later renamed to Jejungwon when it became a teaching hospital, and still exists today as Yonsei University’s Severance Hospital.
Allen’s work was of course only possible because this was the post-1876 era (i.e. very different from what early catholics faced) and the royal court’s primary interest was now in utilising the skills of these missionaries rather than suppressing the religion. In addition Allen’s success encouraged the court to support other skilled missionaries, such as Mary Scranton’s foundation of Ewha Womans (sic) University. These missionaries were able to cautiously engage in evangelic work as part of their work, albeit not openly until the court gave permission in 1898.
Many of the people, who were already from elite backgrounds, educated in the systems initiated by the missionaries went on to further study abroad to the US and other countries. More than a few became prominent figures in the liberation struggles, such as Seung-man Rhee, Man-shik Jo, etc. This concentration of prominent figures in protestant circles had a significant impact in the relative growth of the two churches post-liberation.
Contrast in early spread
There is an interesting contrast between both religions, in part due to differences in circumstance and in part due to the difference in approaches. As Ig-Jin Kim mentions:
First, most pioneer Catholics were learned Koreans while the early Protestant faith found its adherents among the humbler classes. Second, the Catholics did not teach the Bible enough while the Protestants spread the Bible. Third, the Catholics tried to take advantage of political and military capabilities of world powers as factors for missionary enterprise while the Protestants tried first to meet the conditions of the Korean government. Fourth, the early Catholic missionaries lived together with Korean believers and established the unity of the church while the Protestant missionaries lived separately and evangelized fervently, on the one hand, and trained Koreans so that they could evangelize and lead the Korean churches, on the other.
To a large degree catholicism had quite a lot of similarities with the already prevalent confucianism of late Joseon than protestantism did and thus could spread through appealing to already prevalent sensibilities whereas protestantism gained credibility through work that was strictly speaking outside of missionary work but was crucial in enabling missionary work to take place.
During the japanese period both religions suffered from oppression such as the forced visits to shinto shrines.
Continued in part 2