Greek theologian. His family had long been involved in fiscal administration in Damascus and he succeeded his father at the court of the Caliph. He resigned his office, perhaps as early as 706, and became a monk near Jerusalem (traditionally at the monastery of St Sabas), and also a priest. He was a strong defender of images in the Iconoclastic Controversy.
His most influential prose work, the Fountain Head of Knowledge, consists of three parts: a textbook of Logic (‘Dialectica’), a list of heresies (ending with Islam), and ‘De Fidei Orthodoxa’. In ‘De Fidei Orthodoxa’ John presents an outline of the Christian faith, mainly in the form of excerpts from earlier authors. Its four sections deal with the Trinity; creation, human nature and providence; Christology; and other subjects. His other great work, the Hiera or Sacra Parallela is a vast collection of biblical and patristic texts, arranged in three books: on God, on human nature, and on the virtues and vices, these last being treated in parallel; hence the Latin name. He was renowned as a preacher, but few sermons survive. His greatest work is perhaps his liturgical poetry. His Canon for Easter (in translation) is familiar as an English hymn (‘The Day of Resurrection!/Earth tell it out abroad’). He exercised great influence on later theology. Feast day, 4 Dec. (in the W., formerly 27 Mar.). See also Barlaam and Joasaph, Sts.