Joan claimed that Archangels had visited her and instructed her to expel the English from France. If we exclude the possibility that this truly was a divine message, then the two possible explanations would seem to be that these were hallucinations or that she made them up. Is there consensus as to which?
/u/thegreenreaper7 has previously discussed the validity/interpretation of Joan of Arc's visions
/u/asinus_docet wrote about the magic of Joan of Arc
/u/sunagainstgold has previously commented on Joan's visions and Joan's charisma to get others to believe her
EDIT: This collection of answers is narrowly focused on Joan of Arc but there are other archival answers about mental illness in the Middle Ages.
u/Holy_Shit_HeckHounds collected several write-ups about Joan of Arc here.
Answers were provided by u/sunagainstgold, u/Asinus_Docet, and u/TheGreenReaper7.
She's a popular historical topic, so there is probably more (a lot more) that could be written.
I'd like to direct you to one of my favorite answers of all time on askhistorians. In response to a question about miracles in Islam, u/GrumpyHistorian wrote an excellent description of various approaches that historians use to derive credible information from sources who describe impossible things. It approaches questions like, is the source confabulating or was the source deceived? What is the source inadvertently telling us? Are there meaningful social realities that differ from literal realities?