if we take the Chinese Christian sect as a given, how would that translate into a christian Japan? I mean while with a stretch I can see a slightly more easy spread of Christians in China (since that wouldn't come with increasingly meddlesome strangers on the coast), I don't see it becoming so culturally dominant to spread to Japan...
to get to the challenge... I don't think you can have a muslim Engand without a more deep penetration of Arabs into Europe, so the simplified answer would be a different battle of Poitiers, but I don't think that would be enough, something must prevent the Caliphate from breaking up (I have absolutely no idea what, I'm not an expert of Arab history). If the Caliphate doesn't break up and manage to expand more the situation for Christianity would be even worse at least most France and Italy would be also conquered and some areas in the Balkans, the Slavs or the Magyars could convert and so on (In the end I think the England wouldn't be conquered). A situation like this surely would kill the vitality of the Christian religion or at least its ability to spread... so Japan is out of the question (at least for me).