"Religion in South Korea is characterized by the fact that an absolute majority of South Koreans (56.9% as of 2015) have no formal membership in a religious organisation...According to national census conducted in 2015:
19.7% Protestantism
15.5% Buddhism
7.9% Roman Catholicin total Christianity is the religion of 27.6% of the Korean population.
(from Wiki)
Lots of work left to do
Many of the Christians in Korea are a lot more serious about it than folks in the West.
Shamanism is also practiced in Korea (like their cousins, the Mongolians). Some folks might be Christian or Buddhist, but still seek out Shamans for certain types of problems or healings.
I think that the 15% Buddhist may be an undercount, but it has been a quarter century since I lived there. I anecdotally thought it was more like half and half, Buddhist and Christian.