Is Christianity a big thing in South Korea?

Because I've just had two theological students from there try to convert me. O_o

Tangentially related (I was doing my Japanese homework at the time) but this is why I love learning languages. I can say stuff like this:

私のホバクラフとは　うなぎで　いっぱいです。

It means my hovercraft is full of eels. XD XD XD