The early historian Eusebius records that Pantaenus, head of the famous catechetical school of Alexandria, travelled east around AD 180 to preach “to the Indians,” and found believers already there. Ancient writers sometimes used “India” loosely, so the exact land is debated — but the report itself is one of the earliest written witnesses to Christianity reaching the East.
Eusebius, Ecclesiastical History V.10 — public domain. The identification of “India” is disputed.
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