The First Ecumenical Council, that of Nicea, met in 325. Fifty-six years later (381), the second one met in Constantinople. Fifty years later (431), the third Council met in Ephesus. But only twenty years elapsed between the third Council and the fourth, which convened in 451 in Chalcedon. Perhaps twenty years between councils seems like a long time; after all, we are used to annual synods. But remember that provincial and regional councils met more often. Between ecumenical councils, which addressed matters of highest importance that pertained to Christendom, twenty years was relatively short. What went on in those twenty...