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.
This is a companion discussion topic for the original content at https://www.worldhistory.org/Council_of_Chalcedon/.