It’s only mentioned once, in all the Bible: “Behold, these shall come from far… and these from the land of Sinim” (Isaiah 49:12).
“The land of Sinim”–where was that? We are not told. Probably Isaiah’s audience didn’t need to be told.
Many scholars have thought “Sinim” was China: the passage suggests some country far away, probably in the East. Others think it was in Arabia or southern Egypt: but I say there was no need to call any of those places “Sinim.”
Maybe the people of Judah in Isaiah’s time had only heard rumors of “Sinim,” and for them the name was synonymous with some terribly exotic place “somewhere out there.” When I was a boy, “Timbuktoo” meant some unspecified country far, far away (although you could find it on a map easily enough).
Or maybe a few travelers and traders had already journeyed from Judah to China, or vice-versa. We think that’s impossible, but of course it wasn’t. It wasn’t even that improbable.
The point is, the Bible’s message, the Gospel of Jesus Christ, is meant for all the world, China included. The Americas are not specifically mentioned in the Bible, but the message is for everybody, whether they ever crossed the ocean or not.
Was “Sinim” China? Certainly it could have been. Just because they didn’t have cell phones and Internet access didn’t make the ancient world’s people stupid. I would hate to put up today’s people against them in any contest of intelligence.