Whispers from Beyond: Medieval Ghosts and Their Messages About the Afterlife - Medievalists.net
Briefly

In the Middle Ages, ghosts were viewed both as potential messengers from purgatory and as deceptive spirits, leading many to cautiously discern their true nature.
Medieval theologians had varied opinions, with some like Aquinas acknowledging ghostly existence but refraining from elaborate speculation on their nature, while others, like James of Clusa, warned that apparitions seen by non-Christians were demons.
The 15th-century writer James of Clusa noted that authentic apparitions were meant only for Christians, suggesting non-Christians encountered demonic impersonations instead.
Cases existed where the faithful had to rigorously test spirits claiming to be deceased. For instance, a benign ghost's inability to recite a prayer revealed it as a demonic entity.
Read at Medievalists.net
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