Antonios 9 was a monk; in 787 he is attested among the readers at the Second Council of Nikaia (the Seventh Ecumenical Council). At the first session, on 24 September 787, he read out a passage from the Ecclesiastical History of Rufinus to the Council: Mansi XII 1034 (Ἀντώνιος ὁ εὐλαβέστατος μοναχὸς). At the fourth session, on 1 October 787, he read out a passage from St John Chrysostom for the hegoumenos Theodosios 19: Mansi XIII 8-9 (Ἀντώνιος μοναχὸς). At the fifth session, on 4 October 787, he read out a document which criticised Severus of Antioch (from the Acts of the Council of Constantinople in 536): Mansi XIII 181. The other readers of passages from the Fathers were all officials of the patriarchate of Constantinople.

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