Stephanos 18 was bishop of Antiocheia in Pisidia; 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 Στέφανος ἐλέει Θεοῦ ἐπίσκοπος τῆς Ἀντιοχέων μητροπόλεως τῆς Πισίδων ἐπαρχίας: Riedinger, p. 782, line 25, p. 824, line 14, cf. p. 893, lines 4-5 (= Mansi XI 644, 672, cf. 692). In the lists of those attending each session he is styled Στεφάνου ἐπισκόπου Ἀντιοχείας Πισιδίας: Riedinger, p. 404, line 5, p. 516, line 28, 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. 893, lines 4-5 (= Mansi XI 692) (ὑπὲρ ἐμαυτοῦ καὶ τῆς ὑπ' ἐμὲ συνόδου). His name occurs in the list printed in Riedinger, p. 734, line 8 (= Mansi XI 696).

Still bishop of Antiocheia in 692, he attended the Quinisext Council at Constantinople and signed the canons: Ohme, Bischofsliste p. 148, no. 33 (= Mansi XI 992) (Στέφανος ἐλέῳ Θεοῦ ἐπίσκοπος τῆς Ἀντιοχέων μητροπόλεως τῆς Πισιδίας ἐπαρχίας).

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