|Arius of Alexandria|
Tertullian (c.160-c.225), sometimes called the "father of Latin Christianity" because of the enormous body of writings he left behind, defended the Trinity as Father, Son and Holy Ghost in his book Adversus Praxean (Against Praxeas). His purpose was to put down the view of Praxeas that, if Christianity were to be monotheistic, then Jesus and the holy Spirit could not be thought of as separate entities. Jesus must have been God incarnate, not a distinct "son of God" who was his own individual.
But Tertullian and others found multiple references to threes in the Old Testament, and they put these forth as prefigurations of the Trinity as it was revealed in the New Testament. Trinitarian Christology was on its way to becoming official doctrine.
Then Arius stepped forward and pointed to the Gospel of John, which read: “You heard me say, ‘I am going away and I am coming back to you.’ If you loved me, you would be glad that I am going to the Father, for the Father is greater than I." (John 14:28) It was clear to Arius that Jesus was subordinate to God in some way, and the Trinitarian view was wrong. Also, if the Son was begotten, then he had a point of origin, as opposed to the Father who always existed.
Many people got involved in the controversy: Origen, Eusebius, Lucian of Antioch, Alexander of Constantinople, Alexander of Alexandria, Socrates of Constantinople, Epiphanius of Salamis. Everyone who was anyone weighed in on just what the Trinity was. Then Emperor Constantine decided he needed clarification. He had legalized Christianity in 313 through the Edict of Milan, and he wanted to make sure Christianity didn't generate controversy. In 325 he called the first Council of Nicaea to resolve the growing issue of Arianism.
|Icon of the Council of Nicaea|
The Emperor, in order to keep things simple, outlawed Arianism and insisted that all his works be burned. Arius was exiled. His ideas lived on under his name, however, especially among the Goths, until the 7th century. His ideas also lived on in the Emperor's son, Constantius II, who ruled after Constantine and was friendly to Arianism, even being baptized on his death-bed by a Semi-Arian bishop!genitum non factum, consubstantialem Patri (begotten, not made, one in being with the Father)
*Who would some day be known as St. Nicholas; yes, that St. Nicholas.