In history class recently, my teacher mentioned that many crusaders believed in a far away mythical Christian Kingdom that would come to their aid in the fight against their enemies. Where did this myth originate from, was the belief widely held?
You're probably thinking of the legends of "Prester John." Not that there isn't always more to be said, but here are a existing few threads on the subject:
Answer from u/judbud123
Answer from u/RexSuciae
https://www.reddit.com/r/AskHistorians/comments/7bcbst/how_did_european_explorers_react_when_they/