Theognios 1 was bishop of Kios in Bithynia; he attended the Third Council of Constantinople (the Sixth Ecumenical Council) in 680 and 681 and was present at all eighteen sessions: Riedinger, pp. 19-787 (= Mansi XI 212-673). In the subscriptions to the statement of the faith, which condemned monotheletism, and to the Council, he is styled Θεόγνιος ἐλέει Θεοῦ ἐπίσκοπος τῆς Κιανῶν πόλεως τῆς Βιθυνῶν ἐπαρχίας: Riedinger, p. 786, line 7, p. 825, line 25 (= Mansi XI 645, 673). He was indisposed when the faith was subscribed at the seventeenth session on 11 September and was represented by his deacon Georgios 19: Riedinger, p. 736, lines 18-20 (Θεόγνιος ἐλέει Θεοῦ ἐπίσκοπος τῆς Κιανῶν πόλεως τῆς Βιθυνῶν ἐπαρχίας ὁρίσας ὑπέγραψα διὰ χειρὸς Γεωργίου διακόνου καὶ οἰκονόμου τῆς κατ' ἐμὲ ἁγιωτάτης ἐκκλησίας διὰ τὸ ἐν ἀσθενείᾳ με εἶναι), cf. Riedinger, p. 737, lines 19-21 (= Mansi XI 646) ("Theognius misericordia dei episcopus ciuitatis Cianorum Bithiniae prouinciae definiens subscripsi per manus Georgii diaconi et dispensatoris sanctae ecclesiae meae pro infirmitate qua detineor"). In the lists of those attending each session he is styled Θεογνίου ἐπισκόπου Κίου: Riedinger, p. 18, line 1, p. 30, line 3, etc. (= Mansi XI 212, 220, etc.).

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