In medieval Christendom's struggle against Islam, the possible existence of a Christian priest-king named Prester John helped fuel European exploration of Africa and India. The legend of Prester John persisted in Europe from the 12th to the 17th century, thanks to a letter (c. 1145) that described a lost Christian kingdom thought to be somewhere in India or Asia. The letter was copied and distributed throughout Europe over many years, and the legend grew to include exotic and enticing details: Prester John was descended from one of the Magi; he had magical abilities and ruled over a great area full of riches and strange creatures; and, perhaps most significantly, he had been victorious over Muslim armies and could be counted as an ally in the battle for territory, resources and souls. Because Europeans had an indistinct knowledge of the lands to the east, Prester John's kingdom was thought to be in India or, in later stories, Ethiopia. The possibility that another Christian kingdom existed on the other side of Muslim territory was a catalyst for European advancement into Asia, Africa and India for centuries; it was one of the reasons Portugal's King João II sent Bartolomeu Dias to find an eastern sea route to India in 1487. Besides its geo-political strategic implications, the story of Prester John was, to devout Christians, also evidence of the universality of the message of Jesus. There has been much speculation and little agreement as to the historical origins of the legend, if any.
Prester John is sometimes called Presbyter John or Presbyter Johannes.