As early as the seventh century the Syrian Christians who followed the views of Nestorius began spreading them eastward, founding sees in Persia and Turkestan, and ultimately spreading as far as Pekin.

There was a certain revival of their missionary activity under the Mongol Khans, but the restricted nature of the language in which their reports were written prevented them from having any effect upon geographical knowledge, except in one particular, which is of some interest.

The fate of the Lost Ten Tribes of Israel has always excited interest, and a legend arose that they had been converted to Christianity, and existed somewhere in the East under a king who was also a priest, and known as Prester John.

Now, in the reports brought by some of the Nestorian priests westward, it was stated that one of the Mongol princes named Ung Khan had adopted Christianity, and as this in Syriac sounded something like "John the Cohen," or "Priest," he was identified with the Prester John of legend, and for a long time one of the objects of travel in the East was to discover this Christian kingdom.

It was, however, later ascertained that there did exist such a Christian kingdom in Abyssinia, and as owing to the erroneous views of Ptolemy, followed by the Arabs, Abyssinia was considered to spread towards Farther India, the land of Prester John was identified in Abyssinia.