Eustathios 12 was bishop of Kelenderis in Isauria; 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 999-XIII 486. In the lists of those present at the first and seventh sessions he is styled Εὐσταθίου ἐπισκόπου Κελενδέρεως: Mansi XII 999 (Βελενδέρεως; the Latin version, at XII 1000, calls him "Eustathio episcopo Cylindereos"), XIII 372-373. Elsewhere he is styled Εὐστάθιος ὁ ὁσιώτατος ἐπίσκοπος Κελενδέρεως or similar: Mansi XII 1110, XIII 149 (Κελεντέρεως), XIII 397. At the second session he supported the traditional veneration of icons as described in statements from pope Hadrian I (Hadianos 1) and condemned iconoclasts: Mansi XII 1110. At the fourth session he subscribed the statements read out from the Fathers in support of the veneration of icons: Mansi XIII 149. At the seventh session he subscribed the statement of the faith adopted by the Council: Mansi XIII 397.

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