Christianity grew very quickly in Korea from about 1880 to 1910 (Rhee Syngman, future first president of South Korea converted during this time, while he was in prison, 1898-1905). Partly, this was due to the opening of Korea to the outside world, and the West, one result of which was that Western missionaries were allowed to preach and teach (and practice medicine, which was an effective way to attract converts, as it had been in China). Partly, this was due to the general crises in Korea due to the loss of the old Korea-China relationship, the intrusion of the Japanese and Western influence, de-Confucianisation, and more. As happens in crises like this, some people turn to religion, and some of them turned to Christianity (and some to other religions - the Donghak Rebellion had religious elements, with the movement giving rise to the (still-surviving) religion of Cheondogyo AKA Cheondoism).
However, while there was rapid growth, this growth was from a base of almost zero. Both Catholicism and Protestantism had entered Korea, brought by Koreans who had converted in China, but the number of Christians in Korea before the entry of Western missionaries was small. By 1910, the number of Catholics had grown to about 73,000 and the number of Protestants was probably similar. The population of Korea was about 17 million at the time, so this approximately 150,000 Christians were only about 1% of the population.
Notably, much of this growth had been in Pyongyang. Almost 80% of Korea's Protestants were in Pyeongan province (i.e., the province containing Pyongyang), and many of them in Pyongyang itself. In 1910, Pyongyang was approximately 30% Christian (which meant that about 10% of Korea's Christians lived in Pyongyang), after the big push to Christianity that was the Great Pyongyang Revival of 1907. The number of churches in Pyongyang led to the city being called "the Jerusalem of the East".
Two big contributions to Pyongyang becoming such a centre of Christianisation were:
Effective missionaries who won converts by helping people, with medical care and otherwise, when Pyongyang was badly damaged during the First Sino-Japanese War, and founded schools.
The economy of Pyongyang was more dependent on independent workers and craftsmen and small farmers than on large landowners. The shortage of arable land in the area meant that there were fewer large rich landowners. This meant that (a) people were more free to make religious decisions, (b) Confucian influence was weaker, with traditional religion being more important, and (c) Pyongyang was under-represented in government and people were motivated to seek change.
One interesting and particularly Korean phenomenon was the role of "Bible women" in spreading Christianity. The Bible women came from the same kind of backgrounds typical of Korean shamans (who were mostly women), and performed a similar social and spiritual role to Korean shamans (but in a Christian context).
After the Japanese annexation, the growth of Christianity slowed. Japan viewed Korean Christianity as a Korean nationalist movement, and discouraged its growth. Despite this, Christianity continued to grow, with the number of Christians doubling in the next 25 years (in the same time, the population of Korea grew by about 30%, so the Christianisation rate reached about 1.35%), still with most Christians being in the north. Pyongyang saw rapid growth accompanying industrialisation, more than quadrupling in population, diluting the Christian population.
For more on Christianity in Korea in this period, see: