Related: Westerners who reached China overland by way of the silk road picked up the name Cathay from central Asia, and westerners who reached China by sea picked up the name China from southeast Asia, and they didn’t realize both names referred to the same country until the late 1500s, when a group of Jesuits living in China started comparing Marco Polo’s account to their own travels and couldn’t find anyone in China who had heard of this “Cathay” place to the north.
https://en.wikipedia.org/wiki/Cathay
The point where they met up with some Saracens in Beijing and asked them if they’d heard of “Cathay”, and they said, “We’re in Cathay right now” basically clinched it, though it took a while for them to convince everyone back in Europe.
Portuguese Jesuits in India sent someone overland to Cathay to check, and along the way he met a returning caravan who told him about the Jesuits they’d met in Beijing a while back.