Anastasios 12 was a priest and monk, probably in the bureau (sekreton) of the patriarch of Constantinople; at the eighth session of the Third Council of Constantinople (the Sixth Ecumenical Council), on 7 March 681, he was among those named by the bishop of Melitene, Theodoros 26, as responsible with him for a statement noting that the previous five Ecumenical Councils had not defined doctrine on the matter of One Will or Two; he denied all knowledge of the document, but was told, with the others, to produce in due course a statement of faith in order to satisfy the Council: Riedinger, pp. 204-208 (= Mansi XI 341-345) (πρεσβύτερος καὶ μοναχὸς; perhaps one of τινες τοῦ σεκρέτου τοῦ ἁγιωτάτου πατριάρχου Κωνσταντινουπόλεως). He was one of the group of suspected heretics who were admitted shortly after the start of the ninth session and who undertook to present a statement of faith: Riedinger, p. 268, p. 276 (= Mansi XI 381, 385). At the tenth session they all presented identical statements, which were accepted: Riedinger, pp. 386-396 (= Mansi XI 449-455). After the Council he was one of the supporters of Makarios 1 who were exiled with him to Rome: Lib. Pont. 81. 14. He is perhaps to be identified with Anastasios the priest who with Leontios 17 was restored to communion by pope Leo II (Leo 16) after accepting the faith of the Sixth Ecumenical Council at Epiphany (683) and anathematising its opponents; he had not been anathematised by the Council: Lib. Pont. 82. 3 (and cf. Anastasios 7).

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