Theodosios 21 was a deacon and monk and notarios and was skeuophylax of the patriarchal oratories; in 787 he attended the Second Council of Nikaia (the Seventh Ecumenical Council) as one of the patriarchal officials who read out documents to the Council; he is attested at the fourth session (1 October 787), where he read out to the Council a passage from the Miracles of Sts Kosmas and Damian by Sophronios and letters from the patriarch Germanos I (Germanos 8) to the bishop of Synnada, Ioannes 138, and the bishop of Nakoleia, Konstantinos 73: Mansi XIII 68 (Θεόδοσιος διάκονος μοναχὸς καὶ σκευοφύλαξ τῶν εὐαγῶν πατριαρχικῶν εὐκτηρίων; the Latin version, at XIII 67, reads "Theodorus") (where he is reading from the Miracles of Saints Kosmas and Damian); XIII 100 (Θεοδόσιος ὁ εὐλαβέστατος μοναχὸς καὶ νοτάριος; the Latin version, at XIII 99, reads "Theodorus") and XIII 105 (Θεοδόσιος ὁ θεοφιλέστατος μοναχὸς διάκονος καὶ νοτάριος καὶ σκευοφύλαξ τῶν εὐαγῶν πατριαρχικῶν εὐκτηρίων; the Latin version, at XIII 106, reads "Theodorus") (where he is reading from Germanos 8).

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