Starting around he 12th century the fable of Prestor John was invented. He was supposed to be the leader of a wealthy Christian kingdom in the far east. This fable continued into the 15th century with Popes and Kings writing to him to help them in their quest to defeat Islam. Despite the centuries of time these leaders had no problem believing that Prestor John still existed and that he would eventually be found.
(Book – The Last Crusade: The Epic Voyages of Vasco Da Gama – Nigel Cliff)