The Legend of Prester John

Prester John is a medieval European myth about India. It arose at a time when there was little to no contact with India, so the legend bears very little resemblance to reality. According to the myth, Prester John was the Emperor of most of India, and the bulk of his subjects were Christians. There was a great deal of hope that Prester John would come with his great army and help European Christians in their struggle against Islam.

Sources:
Church of India: https://orthodoxwiki.org/Church_of_India
The Book of Marvels and Travels (By Sir John Mandeville)
The Search for Prester John: https://www.historytoday.com/miscellanies/search-prester-john