Jed and I both spent some time in Japanese culture, and he may greatly disagree with me. As I see it, Japanese militarism centralized Shinto worship to an unprecedented by putting greater emphasis on the role of the Emperor. After defeat and occupation, state-sponsored Shinto was taboo (and Americans deliberately cut the emperor down to size while leaving him on the throne for stability's sake), but a spiritual void remained. Christianity, ironically, had a strong foothold...in Nagasaki.
So while Christianity took off in Korea, it didn't spread much in Japan. Instead, Japan has sprouted one New Age sect after another, some more harmful than others. Many are politically influential though because they can drive voters to candidates. During the height of Japan's corruption in the 80s and early 90s, companies would donate cash to nonprofit front groups for religious sects (often peace groups) who would push believers to vote for candidates who would in turn grant favors to the donating companies! Erik Baard
