Petros 18 was bishop of Nikomedeia in Bithynia; he attended the Third Council of Constantinople (the Sixth Ecumenical Council) in 680 and 681 and was present at each of the eighteen sessions: Riedinger II 2. 16- 823 (= Mansi XI 209-669). In the subscriptions to the statement of the faith, which condemned monotheletism and to the Council, he is styled Πέτρος ἀνάξιος ἐπίσκοπος τῆς Νικομηδέων μητροπόλεως τῆς Βιθυνῶν ἐπαρχίας: Riedinger II 2. 780, line 19, 823, line 15, cf. II 2. 892, line 7 (= Mansi XI 641, 669, cf. 689). In the lists of those attending each session he is styled Πέτρου ἐπισκόπου Νικομηδείας: Riedinger II 2. 16, line 24, 28, line 28, etc. (= Mansi XI 209, 217, etc.). At the first session he and Solomon 1, speaking on behalf of the patriarch of Constantinople, denied, together with Makarios 1 and Stephanos 17, that the monothelete teachings of Sergios and other patriarchs and of pope Honorios were heretical: Riedinger II 2. 22 (= Mansi XI 213). He and Solomon 1 helped Makarios 1 produce three volumes of statements from the Fathers to support the doctrine of One Will; the texts which they used were dismissed as corrupt by the papal delegates (see Theodoros 22) at the sixth session (12 February): Riedinger II 2. 178 (= Mansi XI 325). At the eighth session (7 March 681) he was one of the bishops named by Theodoros 26 as jointly responsible for a statement noting that previous ecumenical councils had not defined doctrine on the question of Two Wills or One; when asked to state his position, he denied any knowledge of the document; however he and the others were ordered to produce in due course written statements of their faith: Riedinger II 2. 204-208 (= Mansi XI 341-345). They were not present when the ninth session opened but were admitted shortly afterwards and declared their readiness to present prepared statements on the faith at the next session: Riedinger II 2. 268, 276 (= Mansi XI 381, 385). Their statements, all identical, were presented to the tenth session: Riedinger II 2. 390-396 (= Mansi XI 449-455). 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 II 2. 892, line 7 (= Mansi XI 689) (ὑπὲρ ἐμαυτοῦ καὶ τῆς ὑπ'ἐμὲ συνόδου). He was still bishop of Nikomedeia in 692 when he attended the Quinisext Council and signed the canons: Ohme, Bischofsliste, p. 146, no. 18 (= Mansi XI 989) (Πέτρος ἀνάξιος ἐπίσκοπος τῆς Νικομηδέων μητροπόλεως τῆς Βιθυνῶν ἐπαρχίας).

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