If I remember correctly (I don't recall much of my undergrad Japanese History Course), but by the end of the 16th century, Japan had one of the largest Christian communities outside of European rule. In fact, the Christian community in Japan was a vibrant community, until it was crushed and driven underground by the Tokugawa shogunate. Today, only one percent of Japanese in Japan are Christian.
But what would have happened if Christianity was more successful in Japan? To the point that either Japan or a large part of Japan was Christianized by Christian missionaries? How would that change Japan and its relationship with the world?