There are ancient Pagodas in Western China that were actually Nestorian Christian Churches. Those are a better source for the origins of Prestor John. The goddess of mercy in Chines Goodland Buddhismn Guan Yin, is depicted with a child and may, in fact be the virgin Mary
IIRC there is also archaeological evidence of this sort in Japan.
The Christological heresies followed by Islam stank for the east.
Thank God that the THomas Christians managed to hold out as long as they did.