A patrikios, Kallistos 8 was one of the leading notables under Constantine V (Konstantinos 7), and is described as a good speaker and listener (presumably meaning that he was a good adviser to the emperor and also one who faithfully carried out orders): Vita Steph. Iun. 129, 3ff. (1121D) (ἕνα τῶν αὐτοῦ περιδόξων μεγιστάνων λογιώτατον καὶ πρὸς τὸ λέγειν καὶ ἀκούειν ἐπιτήδειον, Κάλλιστον τοὔνομα, καὶ τῇ τοῦ πατρικίου ἀξίᾳ τετιμημένον); 129, 21, 22 (1124B) (κυριοπατρίκιος and τὸν πατρίκιον). On his name and identity, see Auzépy, n. 213.

A convinced iconoclast, he was sent by the emperor to Stephen the Younger (Stephanos 2) on Mt Auxentius to persuade him to subscribe the Council of Hieria; he delivered the message orally; it was rejected by Stephanos 2, and Kallistos 8 returned to the emperor to report; he was then given officers and soldiers and sent back to Mt Auxentius (they went ἅμα τῷ προπεμφθέντι πατρικίῳ Καλλίστῳ) to seize Stephanos 2 and imprison him in the monastery there; after a few days Stephanos 2 was released, when the soldiers were required for the emperor's campaign against the Bulgars: Vita Steph. Iun. 130, 7-131, 12 (1121D-1125B). Kallistos 8 then gave the monk Sergios 52 a bribe of one solidus, with the promise of another later, to lay charges against Stephanos 2: Vita Steph. Iun. 131, 14-18 (1125B).

Kallistos 8 was one of the group of leading iconoclasts sent by Konstantinos 7, probably in 762, to interview Stephanos 2, then held in custody in Chrysopolis: Vita Steph. Iun. 142, 5-13 (1140B-C), cf. 143, 27 (1141C) (he and Kombokonon 1 were the heads of the laymen in the group - οἱ δὲ περὶ Κάλλιστον καὶ Κομβοκόνωνα ὄντες τῆς συγκλήτου). On their return to Constantinople it fell to the laymen in the group to explain to Konstantinos 7 that the mission had failed to influence Stephanos 2: Vita Steph. Iun. 146, 5-8 (1145A). Stephanos 79 is said to have entered the service of Kallistos 8 (he became his λειτουργός): Vita Steph. Iun. 147, 25-148, 1 (1148B). Some time after the plot with Sergios 52 against Stephanos 2 and after the death of Stephanos 2 (764, 765 or 767), Kallistos 8 and another plotter, Aulikalamos 1, were threatened with exposure by a servant girl of Anna 6 (Anonyma 32), who had given false information in return for promises which had not been kept; to keep her from disclosing their role in the plot, they arranged for her to marry a notarios of Aulikalamos 1 at Nikomedeia: Vita Steph. Iun. 175, 3-9 (1184A).

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