Arianism receives its name from Arius, a Christian priest of Alexandria who taught that the Son of God, the second person of the Trinity, is not God in the same sense as the Father. He believed that the Son of God did exist before time, but that the Father created him and therefore the Son of God is not eternal like the Father. Arius was accustomed to say of the Son of God: “There was a time when he was not.”

When the bishop Alexander opposed Arius, he took his case to Eusebius, bishop of Nicomedia, who had the ear of Emperor Constantine the Great. In order to put an end to the disputes that arose because of Arius’s teaching, Constantine called for a general council that met at Nicaea in 325 c.e. Arius and his followers were condemned by 318 bishops at Nicaea who also drew up a creed laying down the orthodox view of the Trinity.

Known as the Nicene Creed, it states that the Son of God is “God from God, Light from Light, True God from True God, begotten not made, consubstantial with the Father ...” The term used to express the idea that the Son of God is consubstantial, or of the “same substance,” as the Father, homoousios, became a rallying cry for the orthodox side, expressing the unity of nature between the Father and the Son of God.

The years following the Council of Nicaea were turbulent, in which many groups opposed the teaching of the council. The reason Arianism continued to exert influence after its condemnation was due in large part to the emperors of this period. Some were openly sympathetic to this heresy, while others—wanting political peace and unity in the empire—tried to force compromises that were unacceptable to those fighting for the Son of God’s equality with the Father.

Some bishops were orthodox in their understanding of the Son of God as truly God, but they were opposed to the word homoousios because they could not find it in scripture. Others feared that the word smacked of Sabellianism—an earlier heresy that had made no ultimate distinction between the Father and the Son of God, holding that the divine persons were merely different modes of being God.

The defender of the orthodox position was Athanasius, the successor to Alexander in the diocese of Alexandria. Athanasius vigorously opposed all forms of Arianism, teaching that the Son must be God in the fullest sense since he reunites us to God through his death on the cross.

One who is not truly God, he argued, cannot bring us a share in the divine life. Athanasius went into exile five times for his indefatigable defense of Nicaea. A synod held under his presidency in Alexandria in 362 rallied together the orthodox side after clearing up misunderstandings due to terminology.

This synod, along with the efforts of the Cappadocians, theologians who took up the banner of orthodoxy after Athanasius’s death, paved the way for the Council of Constantinople in 381, which reaffirmed the Nicene Creed and its condemnation of Arianism.