This is my "at least" answer:

Nestorian Christianity spread at least as far as Mongolia (Ghengis Khan's wife Sorghaghtani Beki was a Nestorian). Abyssinia (modern day Ethiopia and the Horn of Africa) was a Christian empire in the 7th century CE.

Christian individuals pretty much made it everywhere except major areas of inland Africa (traders were in contact all along the Eastern Coast. But, as far as permanent communities, China was probably the farthest away with communities lasting centuries scattered throughout their empire as Christianity flourished under the relatively open Tang Dynasty.