The Council of Chalcedon was called in 451 CE by the Roman Emperor Marcian (r. 450-457) to settle debates regarding the nature (hypostases, "reality") of Christ that had begun at two earlier meetings in Ephesus (431 CE and 439 CE). The question was whether Christ was human or divine, a man who became God (through the resurrection and ascension) or God who became a man (through the incarnation, "taking on flesh"), and how his humanity and divinity affected his essence and being, if at all.
More about: Council of ChalcedonDefinition
Timeline
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325 CERoman Emperor Constantine I calls the Council of Nicaea.
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381 CEThe First Council of Constantinople adds to Nicene Creed.
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431 CEFirst Council of Ephesus.
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439 CESecond Council of Ephesus.
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8 Oct 451 CE - 1 Nov 451 CEFourth Ecumenical Council takes place in Chalcedon, determining that Jesus Christ had two distinct natures and wills, perfectly united.
