Leo 58 was bishop of Sebaste in Phrygia Pacatiana; 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 (he is not recorded among those attending the first session on 24 September 787). In the list of those present at the seventh session he is styled Λέοντος ἐπισκόπου Σεβάστης: Mansi XIII 372. Elsewhere he is styled Λέων ὁ ὁσιώτατος ἐπίσκοπος Σεβάστης or similar: Mansi XII 1110, XIII 148 (Σεβαστείας). 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 icon veneration: Mansi XIII 148. His name is missing from the list of those who subscribed the statement of the faith adopted by the Council at the seventh session, but he was present at that session and the omission is perhaps due to a defect in the text; his name perhaps is lost in one of the lacunae on Mansi XIII 393 and 396.

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