The map to the left is derived from 2005 census data from South Korea. You see religious affiliation by region. The blue bar represent Buddhists. The purple bar Protestants. And the orange bar are Catholics. The figures do not add up to 100% because a large number of South Koreans do not have a religious affiliation.
The Korean peninsula has witnessed a lot of religious change over the past three generations. Traditional beliefs and mores were stronger in the southern part of the peninsula, so Pyongyang was a major center of Christianity. The division between North and South was accompanied by a migration of Christians from the North to the South. On the order of 1% of the population around ~1950, today Christians form about 30% of the population, with 20% being Protestant, and 10% being Roman Catholic. Buddhists comprise 20% of the population. Most of the gains to Christianity occurred between 1950 and 1995.There has also been a more pronounced growth in Roman Catholicism, which is arguably more prominent than its numbers would warrant. Three of the eight listed potential presidential candidates in 2017 list Roman Catholicism as their religion.
Spatially the patterns make some sense and are not surprising to me. The southwest of the peninsula has traditionally been supporters of the more Left political party, while the southeast has been aligned with the Right. Seoul and its environs to the northeast is arguably the locus for the most Westernized segments of Korean society.
I’m curious what Korean readers, or people who have lived in Korea, have to say about this.