Fifth-century Christianity was a theological battlefield. With the Messalian heretics and their experientialist spirituality on the one side and the intellectualist school on the other, representatives of both extremes found themselves condemned by the Church. In this milieu of subjectivist notions of grace and negative anthropology, there appeared a true mystic, Diadochus, Bishop of Photike in Epiros. His is a theology whose two poles are God's grace and man's ability to cooperate with it by way of discernment of spirits. Diadochus's ability to salvage what was orthodox from the Messalians...