Basilios 15 was bishop of Pergamon; 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 995-XIII 486. In the list of those present at the first session he is styled Βασιλείου ἐπισκόπου Περγάμου: Mansi XII 995. His name appears in the list of those present at the seventh session but the text is defective and his see is given as Assos: Mansi XIII 368. Comparison with the Latin version, at XIII 367) where his name is missing but his see is correctly given ("Olbiano episcopo Elaeae Pergamensium"), proves his presence. Elsewhere he is styled Βασίλειος ὁ ὁσιώτατος ἐπίσκοπος Περγάμου or similar: Mansi XII 1098, XIII 141, XIII 385. 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 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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