Yeah, I'm pretty sure N. Korea bans religion, kind of like the USSR, because the leader is supposed to replace God.

South Korea is something like 50% non-religious, and the rest is split between Buddhist and Christian (a bit more Christian than Buddhist). But at least from my experience, Koreans tend to see religion as more of a social club than actual belief, and in the US, they tend to be business networking places instead of places to actually worship. So even a Christian Korean probably wouldn't be super upfront about it.