Christ and Christianity is well documented in all the kingdoms where it was persecuted and then the subsequent kingdoms had record of those records because one thing about kingdoms is that they don't throw away previous king's records because land records and administrative decrees are stored. Also Christians and Buddhist religions have the tradition of holding conferences that are well documented and happen after every few centuries. As an Indian I have read about records of the Buddist conferences held by various kings since after Buddha. Do I think Buddha is God. No. But he was pretty influential and impactful. The Christian conferences are also there where major theology points are discussed and confusions clarified. The records of thoe conferences exist