Actually, Sayaka introduced this article to me -- "What Makes the Korean Church Grow?" The author of this article of course considers numerous spiritual factors, like providence of God, for that growth. But he also offers several non-theological elements that have furthered the expansion of Christianity in Korea, which I think are objective observations, and want to share with you. Based on this article, I've summarized these non-theological factors below.
1. Formal Protestant missions in Korea started in 1884 with the arrival of American Presbyterian missionaries including Horace Allen, Horace Underwood and Henry Davies. Unlike Catholicism, Protestant Christianity came to Korea at a time of total breakdown in the nation's social, political, and religious life. The five hundred-year-old Yi dynasty was losing its independence to the rising empire of Japan. As the official faith of doomed dynasty, Confucianism was becoming thoroughly discredited; Buddhism had been in decline even longer. As the traditions of centuries were falling in clusters, many Koreans, in despair, turned with hope to the new strong, self-confident faith of the Christians. In such circumstances the church's association with the West was not the liability it has been in other parts of the Third World. It was more an asset. For the colonialism afflicting the Koreans was not Western but Asiatic. To them the West meant freedom, democracy and progress. Into this vacuum of faith and meaning with its loss of national pride came the Good News. It was the right news at the right time, and it was communicated in the right way.
2. As Confucianism and Buddhism for a time almost disappeared, Shamanism was stronger and more deeply ingrained. Shamanism is a primitive Last Asian animistic faith of nature spirits. It was no match, however, for Christianity. Unlike the higher, organized religions of the world that have been major obstacles to the spread of the Gospel, animism has been more often than not an indication of opportunity rather than resistance. It has been in the religious soil of animism that Christian church planters have reaped their most spectacular harvests. Korea has been no exception.
3. When the Japanese annexed Korea in 1910 and began to harass the church, the authorities found that the church was the one free Korean organization they could not control. Christians were the backbone of the great, non-violent, Korean independence demonstrations of 1919. Again in the years before World War II Christians fought bitterly against compromise with Japanese-imposed Shinto worship and were persecuted for their resistance. Ultimately, this served to identify the church more closely in the popular mind with anti-colonialism and with Korean nationalism. Christianity could no longer be stigmatized as foreign. It had become Korean, sharing the hopes and aspirations of the nation.