Gregorios 51 was a deacon of the Church of the Holy Apostles (either at Constantinople or Nikaia) and a notarios; he attended the Second Council of Nikaia (the Seventh Ecumenical Council) in 787, where he was one of the officials who read out documents to the Council: Mansi XIII 9 (Γρηγόριος διάκονος καὶ νοτάριος), XIII 20 (Γρηγόριος ὁ θεοφιλέστατος διάκονος τῆς ἁγίας μεγάλης ἐκκλησίας τῶν πανευφήμων ἀποστόλων), XIII 73 and XIII 80 (Γρηγόριος ὁ θεοφιλέστατος διάκονος τῶν ἁγίων καὶ πανευφήμων ἀποστόλων). He read from the works of the Fathers at the fourth session, on 1 October 787: Mansi XIII 9 (Gregory of Nyssa), 20-21 (the Martyrdom of Anastasios the Persian), 73 (Theodoret of Kyrrhos), 80 (Basil).

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