If you combine Catholics and Protestants, South Korea’s Christian population passed the 50% mark a few years ago. This makes them the second predominantly Christian nation in Asia, after the Philippines.
Best info I have on that subject is from the CIA World Book of Facts, with census data from 1995:
Christian 26.3% (Protestant 19.7%, Roman Catholic 6.6%), Buddhist 23.2%, other or unknown 1.3%, none 49.3% (1995 census)
How few years, I guess, is the question. They would have to have about doubled in the past 12 years... Possible, I suppose.