We're doing a brief review of all 21 Ecumenical councils.
Start here or this may not make sense.
Ok, let's look at the next five more eccumenical councils.
You ready? Here we go!
4. COUNCIL OF CHALCEDON (451)
The Council of Chalcedon—150 bishops under Pope Leo the Great and the Emperor Marcian -- defined the two natures (Jesus Christ is one person and has two natures, Divine and Human) in Christ against Eutyches, who was excommunicated. A bit more on this Eutyches fella. He was a boss moster at the Council of Ephesus, vehemently opposing to the teachings of Nestorius. Unfortunately his condemnation of Nestorianism led him to an equally extreme, although opposite view, which lead to his being denounced as a heretic himself (ooh, so close!)
Read more here.