Japan clamped down on all that pretty quickly, even before Tokugawa-era self-isolation, and as such, Christianity never got a hold in the mainstream there (though later the handful of Japanese Christian converts had a role to play in establishing western-style universities), unlike in Korea, where American evangelism had a stronger influence.

I have wondered whether some of the difference between Japanese and Korean pop music comes down to this: Japanese pop has largely drawn from jazz and jazz-fusion, which is not strongly tied to Christianity, whereas K-pop has more influences from soul and R&B, genres with roots more immediately in Black American gospel music.