Always found it interesting how fertile of a ground Korea was for Christianity. Some places like Japan have been the target of missionaries for centuries with minimal results.
Seoul has a large mosque built in the 1970's by the Saudis. I don't know what their attendance is like.
In Asia, (South) Korea is second to the Philippines in highest percentage of Christians.
Americans had a long presence in South Korea, with the war.
Japan doesn't even have an ordinary religion like we have. They had "hero" worship--Shinto.
https://en.wikipedia.org/wiki/Shinto
Buddhism isn't a god-centered faith either, as the Buddha was Sidhartha Gautama, an Indian holy man. He INSISTED that he wasn't a god but the folks deified him anyway.