This letter comes from a crucial moment in the Nestorian controversy and partially accounts for why the attempted ecumenical council in Ephesus in July 431 was so fractious. The letter was penned at some point after November 30, 430, when Nestorius had received a letter from Celestine calling for him to recant his Christological views, and prior to Nestorius’s sermons in the cathedral on December 6 and 7, since these are referred to in the postscript added later. It was written in response to a letter from John of Antioch, sent earlier in November 430, in which John had distanced himself from Nestorius’s views and encouraged him to comply with the summons from Celestine and Cyril to confess that Mary was Theotokos, “bearer of God.” In other words, although John is usually portrayed as an ally of Nestorius, and eventually became such, at this point he was effectively siding with Celestine and Cyril, leaving the bishop of Constantinople without an ally among the powerful sees of Christendom.