Konstantinos 21 was vir gloriosissimus, apo hypaton, patrikios and kourator of the basilikos oikos of Hormisdas; he was one of the laymen who attended the Third Council of Constantinople (the Sixth Ecumenical Council) in 680 and 681; he attended all eighteen sessions of the council, from 7 November 680, to 16 September 681; in those sessions at which the emperor was present (sessions one to eleven and eighteen) he is named sixth of the laymen in attendance, and at the others he is named first of the four laymen present: Riedinger, p. 14, line 25, p. 26, line 26, p. 36, line 7, p. 160, line 28, p. 170, line 21, p. 180, line 25, p. 190, line 22, p. 262, line 22, p. 278, line 22, p. 400, line 22, p. 514, line 16, p. 570, line 1, p. 628, line 17, p. 664, line 26, p. 682, line 25, p. 706, line 4, p. 752, line 21 (= Mansi XI 209, 217, 221, 229, 317, 321, 328, 333, 380, 388, 457, 520, 549, 584, 604, 612, 621, 625).

He is styled Κωνσταντίνου τοῦ ἐνδοξοτάτου ἀπὸ ὑπάτων πατρικίου καὶ κουράτωρος τοῦ βασιλικοῦ τῶν Ὁρμίσδου οἴκου; the old Latin version printed in Riedinger, p. 15, line 24, etc. (= Mansi XI 210, etc.) calls him "Constantino glorioso exconsule patricio et curatore diuinae domus Hormisde". He and three other laymen represented the absent emperor at sessions twelve to seventeen (ἐκ προσώπου αὐτοῦ παρόντων καὶ ἀκροωμένων Κωνσταντίνου ... with Anastasios 10, Polyeuktos 2 and Petros 23): Riedinger, p. 514, lines 14-21, pp. 568-569, lines 27ff., p. 628, lines 15-22, p. 664, lines 24-31, p. 682, lines 23-30, p. 706, lines 3-8 (= Mansi XI 520, 549, 584, 604, 612, 621). They were named by the emperor at the close of the eleventh session as his representatives (ἐκ προσώπου τῆς ἡμῶν εὐσεβείας: Riedinger, p. 512, line 21 = Mansi XI 517): Riedinger, p. 512, lines 19-21 (= Mansi XI 517) (Κωνσταντῖνον καὶ Ἀναστάσιον τοὺς ἐνδοξοτάτους πατρικίους, Πολύευκτον καὶ Πέτρον τοὺς ἐνδοξοτάτους ἀπὸ ὑπάτων).

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