Anastasios 21 was bishop of Knossos in Crete; in 787 he attended the Second Council of Nikaia (the Seventh Ecumenical Council) and is attested at the fourth and seventh sessions, on 1 October and 13 October 787; he was perhaps present at the last five sessions, from 1 October to 23 November 787: Mansi XIII 145-486. In the list of those present at the seventh session he is styled Ἀναστασίου ἐπισκόπου Κνωσσοῦ: Mansi XIII 369. Elsewhere he is styled Ἀναστάσιος ἐπίσκοπος Κνωσσοῦ or similar: Mansi XIII 145, XIII 389. At the fourth session he subscribed the statements read out from the Fathers in support of the veneration of icons: Mansi XIII 145. At the seventh session he subscribed the statement of the faith adopted by the Council: Mansi XIII 389.

Anastasios 21 was bishop of Knossos (Knosia, in Crete), he was the addressee of a letter from Theodoros 15 (Theodore the Stoudite), written before 814, possibly shortly before January 809, in which Theodoros 15 sought his advice on how to be a good head of a monastery; Theodoros alluded to τὴν ἀρχιερωσύνην σου; he is styled τῆς ὁσιότητός σου and addressed as e.g. ὦ ἱερώτατε πάτερ, ὦ ἱερὰ κεφαλή, πατέρων ἄριστε and ὦ πανσεβάσμιε πάτερ: Theod. Stud., Ep. 11 (addressed Ἀναστασίῳ ἐπισκόπῳ Κνωσίας).

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