Leo 56 was bishop of Porthmos in Euboea; in 787 he attended the Second Council of Nikaia (the Seventh Ecumenical Council) and was probably present at the last seven sessions, from 26 September to 23 November 787: Mansi XII 1110-XIII 486. In the list of those present at the seventh session he is styled Λέοντος ἐπισκόπου Πορθμοῦ: Mansi XIII 369. Elsewhere he is styled Λέων ὁ ὁσιώτατος ἐπίσκοπος Πορθμοῦ or similar: Mansi XII 1110, XIII 145, XIII 392 (Πορθμοῦ). 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 1110. At the fourth session he subscribed the statements read out from the Fathers in support of the veneration of icons: Mansi XIII 145. At the seventh session he subscribed the statement of the faith adopted by the Council: Mansi XIII 392.

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