An important but underrecognized aspect of the reception of Nicaea I (325) at subsequent ecumenical councils is the fifth-century concept of the “sufficiency of Nicaea.” The Acts of Ephesus I (431) contain a decree (the so-called Canon 7.00 of Ephesus) that prohibits anyone from writing a different creed as a rival to the one promulgated at Nicaea I. The bishops present at Ephesus I who put forward this canon did not seem to be aware of the document associated with Constantinople I (381)—what we today consider the final version of the Nicene Creed. When the bishops later became aware of that document, they faced the thorny problem of how to understand it in the light of a decree about the sufficiency of an earlier creed from 325. Further complicating this question was the fact that when Emperor Marcian called the Council of Chalcedon (451), he wanted the bishops to write a new creed, so that he could see himself as a new Constantine. This paper utilizes the mammoth work of Richard Price in translating the acts of the ecumenical councils between 2005.00 and 2020, and builds on several of his intuitions, as well as extending a recent argument by Mark S. Smith (The Idea of Nicaea in the Early Church Councils, AD 431-451, OUP, 2018). The paper uses the bishops’ statements on the floor at Chalcedon and the full (five-page) text of the Chalcedonian Definition (as opposed to the single paragraph often regarded to be the Definition) to argue for three key points: 1) That the Chalcedonian Definition affirms both the document from 381.00 (the final Nicene Creed) and the sufficiency of the Creed of Nicaea from 325.00 by insisting that the later document is not a different creed but is “the same faith” as the Creed of 325. 2) That along with the Creed, the Chalcedonian Definition affirms a particular way of interpreting the Creed—that of Cyril of Alexandrian in contrast to that of Nestorius. 3) That the Chalcedonian Definition is not a quasi-creedal statement focused primarily on a formula (two natures united into one person), but instead a commentary on the Creed of Nicaea that provides an authoritative way to understand that Creed. Seeing the Chalcedonian Definition in this way enriches our understanding of patristic Christology and of the place of the Creed we celebrate this year as the premier summary of the biblical account of Jesus Christ.