This is a Council of Ephesus What If. Here's the background:
-August, 430 Pope Celestine gives Nestorius 10 days to accept Cyril’s Christological teaching.
-November 19th, 430 Nestorius convinces Theodosius II to hold a general council to settle the Christological dispute.
-June 22, 431 Cyril holds Council of Ephesis before the Antiochene faction arrives, and anathematizes Nestorius’ teachings and excommunicates him
-June 26, 431 John of Damascus and pro-Nestorius, Antiochene Bishops arrive and hold their own council. They excommunicate Cyril and Memnon, anathematize Cyril’s Christological doctrine.
-Both Councils send a letter to the Emperor to ratify their proceedings (sort of like how the President signs a bill into law.)
Now, IOTL Theodosius II ratified both proceedings. Cyril then convinced pro-Nestorian Bishops to send letters affirming the proceedings of the original meeting on June 22nd by convincing Theodoret and others that what he taught was the same as the Christology of the Antiochene school. He did this by compromising the language of his statement (as we can see from the Symbol of Union which incorporated what what hammered out at Ephesus–the document essentially conceded everything to the Antiochene school). However, Cyril was able to keep his beloved excommunication of Nestorius. 220 Bishops jump on board. Theodosius II, unhappy to see an agreement among the majority of Bishops to have Nestorius excommunicated, is happy to see that the Christological issue is settled and amenable to every single party (other than just Nestorius.) Theodosius II then withdrew his earlier ratification of the Nestorian/John party and accepts Cyril’s proceedings. The Pope eventually gets word of this and further approves of the proceedings.
What if instead of Theodosius II ratifying the proceedings of both councils as he did initially, he refuses to recognize Cyril's meeting (which originally was his attention anyway) and instead recognizes Nestorius'?
Now, it goes without saying that the Papal legates in July will refuse to recognize Nestorius' council, as it contradicts the decree of the Pope in August of 430. This would put the Pope irreconcilably at odds with the Emperor. Unlike Ephesus II where the same thing pretty much occurred, Theodosius II is going to live another 20 years. Does such an event nip the idea of Papal Primacy at the bud? Does Roman Catholicism resemble something more akin to Eastern Orthodoxy today? Does Cyril hold an Oriental Orthodox schism early? Does Rome join him? Does Roman theology retain a monophysite bias though avoid extreme monophystism which grew after Cyril's death?
Okay, thats enough questions, what say you?
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Edit: The thread title should say Ephesus I, my apologies.