Theodosios 13 was bishop of Sagalassos in Pisidia; 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 998-XIII 486. In the lists of those present at the first and seventh sessions he is styled Θεοδοσίου ἐπισκόπου Σαγαλλάου, Mansi XII 998; and Θεοδοσίου ἐπισκόπου Ἀγαλασοῦ (sic), Mansi XIII 372. In the list of names from the second session he is styled Θεόδωρος (sic) ὁ ὁσιώτατος ἐπίσκοπος Σαγαλλείου: Mansi XII 1107 (the Latin version, at XII 1108, also has "Theodorus"). In the list of names from the fourth session he is styled Θεοδόσιος ἐπίσκοπος Συγαλασσοῦ: Mansi XIII 149. 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 1107. At the fourth session he subscribed the statements read out from the Fathers in support of the veneration of icons: Mansi XIII 149. He is not named in the list of those who subscribed the statement of the faith adopted by the Council at the seventh session, although he was present at that session; the omission is perhaps due to a defect in the text; see Konstantinos 58.

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