Western Europeans came to know about Prester John in a crusading context. Edessa, the first city in the east to be captured by the crusaders, had been reconquered by Zengi of Mosul in 1144. In the following year the historian Otto of Freising met at the papal court Hugh, the Catholic bishop of Jabala in the principality of Antioch, who related how ‘not many years ago a certain John, king and priest, living in the furthest lands of the east beyond Persia and Armenia’, who was a Nestorian Christian, as were his subjects, had made war on the Samiardi, kings of the Persians and Medes, and severely defeated them. He had wanted to help the Church of Jerusalem, but had not been able to find a means of transporting his army across the river Tigris. Bishop Hugh added that the priest-king was said to be descended from the Magi who had come to worship the infant Christ, and also that he had an emerald sceptre.

As Charles Beckingham pointed out, in some manuscripts of Otto's work, the Samiardi appear as Saniardos ‘and may reasonably be supposed to represent [the people of Sultan] Sanjar’, the supreme Seljuk Sultan and ruler of eastern Persia 1118–57. In 1141 he had been defeated by the Kara-Khitai, or Black Cathayans, whose ruler was a pagan, but some of whose people were Nestorian (or more properly Chaldean) Christians. This event had been transmuted into a story which boosted Christian morale by claiming that beyond the lands of Islam there were powerful Christian rulers who were potential allies of the Franks in the Crusader States.

The West in the 1140s was cut off from direct contact with most of Asia and Africa by Islamic rulers whose power extended from central Anatolia to southern Spain. Although western merchants traded with Muslims, they were not normally allowed to travel beyond the coastal ports. The alternative route to Asia lay through the South Russian steppes, but seems to have been considered impractical by Christian travellers before the rise of the Mongol Empire.

Western scholars knew about the lands beyond Islam from the writings of Greek and Roman authors, which they read chiefly in the epitomes of late classical and early medieval scholars such as Orosius, Martianus Capella, Macrobius and St. Isidore of Seville.