There is apparently evidence of Christianity having come to Japan long before Francis Xavier, in the form of Nestorian Christianity (see here, though I can't vouch for the accuracy of the claims made there). While historically it obviously didn't exactly take charge of the Japanese state... well, what if it did? Differing court politics in Tang China and Asuka period Japan could have lead to the Nestorians making deeper inroads in Japan.
Suppose that Nestorian Christianity winds up taking the spot that Buddhism did historically; what are the likely long term consequences?
Suppose that Nestorian Christianity winds up taking the spot that Buddhism did historically; what are the likely long term consequences?