Ioannes 138 was bishop of Synnada in Phrygia Salutaris; addressee of a letter from the patriarch Germanos I (Germanos 8) of Constantinople (715-730) concerning one of the bishops in his jurisdiction, the bishop of Nakoleia, Konstantinos 73; Germanos 8 gave an account of the arguments which he had sent to Konstantinos 73 in response to the latter's doubts about the veneration of icons; Konstantinos 73 failed to pass the original letter on for Ioannes 138, who was his metropolitan, to read: Mansi XIII 100-105 (letter from Germanos 8). Ioannes 138 wrote to Germanos 8 to report that Konstantinos 73 had not sent a copy of the letter to him, provoking Germanos 8 to write an angry letter to Konstantinos 138: Mansi XIII 105 (Ἰωάννης ὁ θεοφιλέστατος τῆς Συναδέων μητροπολίτης). The date was evidently around the time when iconoclasm became a problem, c. 726/730.

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