Kosmas 1 was bishop of Synnada in Phrygia Salutaris; in 681 he attended the Third Council of Constantinople (the Sixth Ecumenical Council) and was present at the last eight sessions (sessions eleven to eighteen, from 20 March to 16 September): Riedinger, pp. 404-824 (= Mansi XI 457-672). In the subscriptions to the statement of the faith, which condemned monotheletism, and to the Council he is styled Κοσμᾶς ἐλέει Θεοῦ ἐπίσκοπος τῆς Συναδέων μητροπόλεως τῆς Φρυγῶν Σαλουταρ͂ων ἐπαρχίας: Riedinger, p. 782, line 21, p. 824, line 9, cf. p. 892, line 35 (= Mansi XI 644, 672, cf. 692). In the lists of those attending each session he is styled Κοσμᾶς ἐπισκόπου Συνάδων: Riedinger, p. 404, line 4, p. 516, line 27, etc. (= Mansi XI 457, 520, etc.). He was among the signatories to the copy of the definition of the faith sent to the bishop of Rome, Agatho 1, signing on behalf of himself and the bishops under him: Riedinger, p. 892, lines 35-36 (= Mansi XI 692) (ὑπὲρ ἐμαυτοῦ καὶ τῆς ὑπ' ἐμὲ συνόδου ὁμοίως ὑπεσημηνάμην). His name occurs in the list printed in Riedinger, p. 734, lines 3-4 (= Mansi XI 696), with his see (misspelt) and province.

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