Theodosios 19 was hegoumenos of the monastery of Hagios Andreas the Nesios; in 787 he attended the Second Council of Nikaia (the Seventh Ecumenical Council) and was present at the fourth session, on 1 October 787, where he produced a book of works by John Chrysostom and asked for a passage to be read; he is styled Θεοδόσιος ὁ εὐλαβέστατος ἡγούμενος μονῆς τοῦ ἁγίου Ἀνδρέου τοῦ Νησίου: Mansi XIII 8. He is not named among those at this session who subscribed the statements read out from the Fathers in support of the traditional veneration of icons; the omission is perhaps an error. The location of his monastery is unknown.

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