Hmmm, again, depending on the kind of Christianity and how it interacts with Japanese culture will be the big question, I think then. If they get a form that focuses on evangelizing the entire planet, maybe it might be possible for Japanese missionaries to reach out to China and Korea, which could have an interesting effect for Christianity in Korea. One academic article I read about Christianity here mentioned that two of the reasons it spread so widely in Korea were that 1) it wasn't seen as a foreign religion, because Korean converts from China were the ones who first started spreading it during the Joseon dynasty, and 2) during the Japanese occupation it was associated with Korean patriotism, because they refused to worship the Japanese emperor, though for theological rather than patriotic reasons.
As for Japan, if they were to have THAT particular brand of Christianity and the associated evangelism, perhaps that could have Japan breaking out of isolation earlier, and god knows what effects could happen depending on where their ships land. Anything from a Japanese Hawaii or Indonesia could happen.
Of course, if you start the ball rolling that early, that butterflies away god knows how much of the 19th and 20th century as we know it.
But again, it depends on what aspects of Christianity they pick up. If different parts of Japan pick up different versions, say Catholicism one area and various Protestant versions elsewhere, that could be bad for Japan and cause a 30 Years war-ish scenario in Japan, or at least exacerbate the shogun/warlord situations. It really depends on the details here.