In the first few centuries after the birth, death, resurrection, and ascension of Christ, the hot topics of conversation within the Church often centered on these questions: who is Jesus? What is Jesus? How do we make sense of all of the things he said and did? He healed the sick, fed multitudes from a few loaves and fish, even raised the dead, even rose from the dead himself. He was clearly a prophet, perhaps the greatest of prophets, the Messiah who was to come and restore Israel. But he also said certain things, like “I and my Father are one,” and “He who has seen me has seen the Father.” Was… was he claiming to be equal to God somehow, or to be God Himself? How could Jesus be God if there is only one God? Could God become a human being and still be God? And even if Jesus were God, how would we reconcile that with him saying things like, “Why do you call me good? No one is good but God alone,” or with the Gospels saying that Jesus grew in wisdom (does God need to learn anything)? Is Jesus a man? Is he God? Both? Neither? Something else? How do we express his identity?
Many people tried many solutions to the problem, but most of them tended to fall on one side or the other of the “God or man” equation. Docetists said that Jesus was really God, but only appeared to be human (“Docetist” from the Greek dokein meaning “to appear, to seem”); he didn’t really suffer or die, but sort of went through the motions, his human form being a mere suit of clothes or mirage. Adoptionists said that Jesus was really a human being, but was granted special favor by God and elevated or “adopted” at the moment of his baptism in the River Jordan (“This is my beloved son in whom I am well pleased”). Different Gnostic groups took some things they read in Neo-Platonic writers and constructed a whole mythos in which human souls were trapped in bodies by an evil creator god (the Demiurge), and Jesus was a spirit who had come to free them by giving them the knowledge that they were imprisoned (“Gnostic” from Greek gnosis meaning “knowledge”).
None of these seemed right. The general sense, gathered from Sacred Scripture, the apostolic tradition of the Church, and the teaching of the bishops around the world, was that Jesus had to be somehow both God and man. But how could that be? Many more made attempts. Some said that God was really one, but appeared in different forms at different times: sometimes as Father, sometimes as Son, sometimes as Spirit. Various ideas had this basic concept, and became known as monarchianism ( Greek mono + arche = “one beginning/origin/power”), or modalism (as in, “God appears in different modes: Father mode, Son mode, Spirit mode”), or patripassianism (Latin “pater” + “passio” = “The Father suffering,” meaning that though it appeared a different person, the Son, was suffering, the Son is just a mode of the Father, so it was really the Father who suffered on the cross). There were others, all falling to the same problem of not respecting both the unity of God and the distinction between the Father and the Son.
Many of these teachers began trying to make use of philosophical terms to help explain themselves, terms like substance, nature, and person. Several challenges stood in the way of this, though. One, the eastern part of the empire was largely Greek-speaking, while the west was Latin-speaking; add to this that the Greek theologians were using more terms than their Latin counterparts, and problems abound. The Latins heard ousia and physis and hypostasis and prosopon and tried to cram them into persona, natura, and substantia. It also didn’t help that the Greeks couldn’t decide what their terms meant—they had a bad habit of using these words without defining them. One person uses physis to mean “nature/essence/what-it-is,” while another uses it to mean “center of subjectivity/who-it-is.” Confusion abounded.
Then, a priest from Rome named Arius began teaching in the Egyptian city of Alexandria that the Son was distinct from the Father, but that he was a creature, the greatest of all creatures and nearly a god himself, but that “there was a time when the Son was not”: he was not eternal; he was not God. But, being that he died for our sins and was glorified by God, he was still worthy of our veneration.
This idea became very popular, especially among certain influential Roman nobles, and the Germanic barbarians living on the borders of the empire. Much of the Church in the Eastern part of the empire took to this new teaching; as St. Jerome wrote, “The world awoke and groaned to find itself Arian.” The western part of the empire still largely held to the traditional view laid out by Tertullian a century before: that Jesus was one person, but a person with two natures, one human and one divine.
Things got bad. Factions sprang up. People were persecuted. Bishops were forced into exile away from their cities.
In 325 AD, the emperor Constantine summoned all the bishops of the world to the resort town of Nicaea and asked them to settle the issue. More than 300 bishops from all over the empire attended, including two legates representing the Pope. This was the first ecumenical (“world-wide”) council in the Church’s history. The bishops discussed, and debated, even fought: St. Nicholas (yes, THAT St. Nicholas) was so furious with Arius that he punched him in the face! The bishops overwhelmingly agreed that Arius was dead wrong. They came up with a summary definition of the Church’s faith in Christ, adding to it at another council held 50 years later in Constantinople. Today we know this definition as the Nicene(-Constantinopolitan) Creed. You say it in Mass every Sunday.
(Tangential epilogue: People sometimes wonder, if the Creed is supposed to be the most basic and fundamental expression of the Christian faith, why is there no mention of the Eucharist, expressing the Church’s belief that it is truly the Body and Blood of Christ? The answer is simple: nobody disputed this point at the time. Creeds and council declarations address the points being controverted at the present time. The Eucharist as the Real Presence of Christ? That was obvious. The nature of Christ himself? That’s the hard stuff.)