John Cassian, also known as John the Ascetic and John Cassian the Roman (Latin: Ioannes Eremita Cassianus, Ioannes Cassianus, or Ioannes Massiliensis; Greek: Ίωάννης Κασσιανός ό Ερημίτης; c. AD 360 – c. 435), was a Christian monk and theologian celebrated in both the Western and Eastern churches for his mystical writings. Cassian is noted for his role in bringing the ideas and practices of early Christian monasticism to the medieval West.