Sergios 18 was bishop of Leros; in 787 he attended the Second Council of Nikaia (the Seventh Ecumenical Council) and is attested at the fourth and seventh sessions, on 1 October and 13 October 787; he perhaps attended the last five sessions, from 1 October to 23 November 787: Mansi XIII 149-486. In the list of those present at the seventh session he is styled Σεργίου ἐπισκόπου τῆς Λέρου: Mansi XIII 372 (the Latin version, at XIII 371, erroneously calls him "Georgio"). Elsewhere he is styled Σέργιος ἐπίσκοπος τῆς Λέρου or similar: Mansi XIII 149, XIII 396. 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 396.

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