John Cassian, also known as John the Ascetic and John Cassian the Roman (Latin: Ioannes Eremita Cassianus, Ioannus Cassianus, or Ioannes Massiliensis;[2] c. AD 360 – c. 435), was a Christian monk and theologian celebrated in both the Western and Eastern churches for his mystical writings.[3] Cassian is noted for his role in bringing the ideas and practices of early Christian monasticism to the medieval West.