Konstantinos 38 was bishop of Mastaura (in Asia); 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 994-XIII 486. At Mansi XIII 368 and XIII 385 his name is given as Κώνστα; the Latin versions have "Constans" at XIII 386 and "Constantinus" at XIII 367. In the lists of those present at the first and seventh sessions he is styled Κωνσταντίνου ἐπισκόπου Μασταύρων: Mansi XII 994, XIII 368 (Κώστα). Elsewhere he is styled Κωνσταντῖνος ὁ ὁσιώτατος ἐπίσκοπος Μασταύρων or similar: Mansi XII 1098 (Μασταυρίων), XIII 141, XIII 385. At the second session he spoke in support of the traditional veneration of icons as described in statements from pope Hadrian I (Hadrianos 1) and condemned iconoclasts: Mansi XII 1098. At the fourth session he subscribed the statements read out from the Fathers in support of the veneration of icons: Mansi XIII 141. At the seventh session he subscribed the statement of the faith adopted by the Council: Mansi XIII 385.

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