I'm drafting a timeline/conworld off-site about Japan converting to Christianity under an Oda shogunate. Plausibility reservations aside, I want to have Christianity spread more or less peacefully from Japan to Korea (but not to China or further into East Asia, I'm not trying to make a Christianity-wank in the region). How could I do this without imperialism, "crusades" (not that they would happen in a Christian Japan anyway, but some people seem to really like the idea), invasions, colonialism, or otherwise some sort of oppression of Korea by Japan, like what we saw IOTL? My goal is to have Japan and Korea as equal allies (with a shared adherance to Christianity), creating an interesting detraction from OTL and many other timelines (where, respectfully, it seems to be a very common theme for Japan to aggress Korea and for the two to despise one another).
Feel free to answer LibPrime's question up top first if you wish. They posted first, after all; it's only fair.