Theophylaktos 19 was bishop of Apollonias in Bithynia; in 787 he attended the Second Council of Nikaia (the Seventh Ecumenical Council) and was probably present at all eight sessions, from 24 September to 23 November 787: Mansi XII 995-XIII 486. In the lists of those present at the first and seventh sessions he is styled Θεοφυλάκτου ἐπισκόπου Ἀπολλωνιάδος: Mansi XII 995, XIII 369 (called Θεοφίλου, but the Latin version, at XIII 370, has "Theophylacto episcopo Apolloniadis"). Elsewhere he is styled Θεοφύλακτος ὁ ὁσιώτατος ἐπίσκοπος Ἀπολλωνιάδος or similar: Mansi XII 1102, XIII 144, XIII 389. At the second session he supported the traditional veneration of icons as described in statements from pope Hadrian I (Hadrianos 1) and condemned iconoclasts: Mansi XII 1102. At the fourth session he subscribed the statements read out from the Fathers in support of the veneration of icons: Mansi XIII 144. At the seventh session he subscribed the statement of the faith adopted by the Council: Mansi XIII 389.

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