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Theognostos (flourished 9th century) was a Byzantine monk, theologian, and chronicler, coauthor of a report on the situation of the Eastern Church during the turbulent reign of Photius (858–867 and 878–886), the controversial patriarch of Constantinople. This theological chronicle, or “Letter of Appeal,” constituted one of the most important documents on the resulting break (the Photian schism) between the Western and Greek churches. An opponent of Photius, Theognostos supported the reigning patriarch, Ignatius, who was...