Leo 50 was a priest of the Great Church (Hagia Sophia) at Constantinople, he was archon of the monasteries and defensor (ἔκδικος) of the Church (see below); in 787 he attended the Second Council of Nikaia (the Seventh Ecumenical Council) as representative of the metropolitan see of Side (in Pamphylia); he is attested at the second, third, fourth and seventh sessions and was probably present at the last seven sessions of the Council, from 26 September to 23 November 787: Mansi XII 1090-XIII 486. In the list of those present at the seventh session he is styled Λέοντος πρεσβυτέρου καὶ τοποτηρητοῦ τῆς μητροπόλεως Σίδης: Mansi XIII 365. Elsewhere he is styled Λέων πρεσβύτερος τῆς ἁγιωτάτης ἐκκλησίας Κωνσταντινουπόλεως καὶ τὸν τόπον ἐπέχων τῆς μητροπόλεως Σίδης, Mansi XII 1090-91; Λέων χάριτι Χριστοῦ πρεσβύτερος τῆς ἁγιωτάτης τοῦ Θεοῦ καθολικῆς μεγάλης ἐκκλησίας καὶ ἄρχων τῶν μοναστηρίων καὶ ἔκδικος τῆς ἁγιωτάτης τοῦ Θεοῦ ἐκκλησίας, XIII 136; and Λέων πρεσβύτερος τῆς ἁγιωτάτης τοῦ Θεοῦ μεγάλης ἐκκλησίας καὶ ἐκ προσώπου τοῦ θρόνου τῆς Σιδήτων μητροπόλεως, Mansi XIII 381. At the second session he spoke in support of the traditional veneration of icons as described in statements from pope Hadrian I (Hadrianos 1) and condemned iconoclasts: Mansi XII 1090-91. At the third session he accepted as orthodox the statement of the faith contained in the letter from the eastern patriarchs; he is styled Λέων ὁ ὁσιώτατος ἐπίσκοπος (sic) καὶ τὸν τόπον ἐπέχων τῆς μητροπόλεως Σίδης: Mansi XII 1150 (the Latin version, at 1149, calls him "Leo sanctissimus praepositus et locum retinens metropoleos Sidae"; presumably "praepositus" is a mistaken expansion of the abbreviation "pr", for "presbyterus"). At the fourth session he subscribed the statements read out from the Fathers in support of the veneration of icons: Mansi XIII 136. At the seventh session he subscribed the statement of the faith adopted by the Council: Mansi XIII 381.

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