Diogenes of Cyzicus (grc|Διογένης ἐπίσκοπος Κυζίκου|Diogenes episkopos Kyzikou[1]) was the metropolitan bishop of the from before 449 to after 451. He participated in the Second Council of Ephesus and the Council of Chalcedon.
He is not to be confused with the 3rd century grammarian Diogenianus or Diogenes of Cyzicus, who wrote the Patria of Cyzicus and is described in the Suda.[2] [3]
As the bishop of Cyzicus, Diogenes was present at the Second Council of Ephesus in 449. At the council, Eutyches attempted to prove his orthodoxy by reciting the original Nicene Creed of 325, and claiming that Canon 7 of the Council of Ephesus forbids any addition or subtraction from that version of the creed. When records from Ephesus II were reviewed at the Council of Chalcedon, Diogenes objected to this claim, noting that additions were already made to the creed at the First Council of Constantinople to counter the heresy of Apollinarianism. He further accused Eutyches of Apollinarianism for not reciting the 381 version of the Creed, specifically the part that says "from the Holy Spirit and Mary the Virgin". A group of Egyptian bishops protested, apparently unfamiliar with the 381 version and considering it a violation of Canon 7 of Ephesus. The ensuing argument over additions to the Creed is relevant to the later Filioque controversy.[4]
In the same complaint, Eutyches anathematized "those who say that the flesh of our Lord and God Jesus Christ came down from heaven". When Eutyches' complaint was reread at the Chalcedon, Eusebius of Dorylaeum pointed out that, while Eutychus correctly denied that Christ's flesh came down from heaven, he did not state from where it came. Diogenes, corroborated by Basil of Seleucia, claimed that they pressed Eutyches at Ephesus, saying "Lord Eutyches, where then did it come from? Tell us" but he didn't comply. The orthodox answer is that Christ took flesh from the Virgin Mary, but this would contradict Eutyches' belief that Christ is not consubstantial with humanity.
Like the other bishops there, Diogenes assented to and signed the proceedings of the Second Council of Ephesus, namely the restoration of Eutyches to his position as archimandrite and the deposition of Flavian and Eusebius. However, some of these bishops later withrew their assents, accusing Dioscorus of threatening them with a mob, forcing them to sign a blank paper, and having his notaries forge statements from them.
Two years later, Diogenes also participated in the Council of Chalcedon. The council began with a reading of the acts of the 448 Home Synod in Constantinople and the Second Council of Ephesus, which Diogenes made several comments on.
Like the other bishops at Chalcedon, he assented to the deposition of Dioscorus and signed the Tome of Leo. When a group of 13 Egyptian bishops wrote a statement of faith to the council, affirming their belief in the ecumenical councils and rejection of heresy without explicitly mentioning Eutyches, Diogenes demanded they anathematize Eutyches and sign Leo's Tome, noting that the council was about Eutyches. They requested to hear Dioscorus' opinion first, arguing that Canon 6 of Nicaea forbids them from making a decision without him, to which Diogenes mocked them saying "How can someone who has no idea what he believes elect a bishop?"