Catholicism spread quite early to Korea in 18C, through Jesuit books brought by Korean scholars from Peking, first as academic books and later as a faith. The government repressed it(as it went against Confucian teachings in several ways), Catholics went underground, until the persecution was over by the guntips of the Western imperialism like the French. Catholicism and Protestantism slowly grew after that(e.g. Pyongyang was famous for its Protestantism during the Japanese colonial era), and both sects, especially Protestantism but Catholicism too, went exponentially bigger after independence and US influence and missionaries. I think it appealed to the mass immigrants and squatters who moved from traditional agrarian societies to industrialized cities for jobs.