Saint John Cassian was born in the Danube Delta in what is now Dobrogea, Romania in about 360. In 382 he entered a monastery in Bethlehem and after several years at the monastery was granted permission, along with his friend St. Germanus, to visit the Desert Fathers in Egypt. They remained in Egypt until 399 except for a brief period when they returned to Bethlehem and were released from the monastery there.
Upon leaving Egypt they went to Constantinople where they met St. John Chrysostom who ordained St. John Cassian as a deacon. He had to leave Constantinople in 403 when St. John Chrysostom was exiled, and eventually settled close to Marseilles where he was ordained priest and found two monasteries, one for women and one for men.
St. John's most famous works are the Institutes, which detail how to live the monastic life, and the Conferences, which provide details of conversations between John and Germanus and the Desert Fathers. He also warned against some of the excesses in Blessed Augustine of Hippo's theology whilst refraining from criticising him by name. For this reason he has sometimes been accused of Semi-Palagianism by the Roman Catholic church.
St. John died peacefully in 435 and his feast day is July 23