Arianism
Arianism was a 4th century heresy named after Arius (c.250-336), a presbyter in Alexandria, Egypt, who taught that the Son of God was not co- eternal and consubstantial with His Father, but rather a created being with a definite origin in time. In Arius's words, "there was [a time] when he (the Son) was not." This led to the calling of the First Ecumenical Council , which condemned it and its author and established the Orthodox doctrine of the Holy Trinity as taught by Arius's chief opponent, St. Athanasius the Great . Though it managed to hang on among some of the Goths and other Germanic tribes in the West, Arianism had vanished by the seventh century. Arianism should be clearly distinguished from "Aryanism", which formed the core of Nazi racial ideology during the twentieth century, and which had nothing whatsoever to do with Arius or his teachings.
The heresy
As stated above, Arius denied the full deity of the preexistent Son of God, the Logos who became incarnate as our Lord Jesus Christ ("the Word (Jesus Christ) became flesh" John 1:14 - NKJV). He held that the Son, while divine and like unto God, was created by God as the agent through whom He created the universe; thus that there was a time when the Son "was not". In explaining his actions against Arius, Alexander of Alexandria wrote a letter to Alexander of Constantinople and Eusebius of Nicomedia (where the emperor was then residing), detailing the errors into which he believed Arius had fallen. According to Alexander, Arius taught: That God was not always the Father, but that there was a period when he was not the Father; that the Word of God was not from eternity, but was made out of nothing; for that the ever- existing God (‘the I AM’—the eternal One) made him who did not previously exist, out of nothing; wherefore there was a time when he did not exist, inasmuch as the Son is a creature and a work. That he is neither like the Father as it regards his essence, nor is by nature either the Father’s true Word, or true Wisdom, but indeed one of his works and creatures, being erroneously called Word and Wisdom, since he was himself made of God’s own Word and the Wisdom which is in God, whereby God both made all things and him also. Wherefore he is as to his nature mutable and susceptible of change, as all other rational creatures are: hence the Word is alien to and other than the essence of God; and the Father is inexplicable by the Son, and invisible to him, for neither does the Word perfectly and accurately know the Father, neither can he distinctly see him. The Son knows not the nature of his own essence: for he was made on our account, in order that God might create us by him, as by an instrument; nor would he ever have existed, unless God had wished to create us.
The Church's response
While Arius developed a following among some Syrian prelates, an Alexandrian synod of some 100 bishops summoned by Bishop Alexander condemned him in 321. He was excommunicated , and fled to Palestine. There he entered into a friendship with Eusebius of Nicomedia. Arius, a proficient writer, produced many compositions in both prose and verse defending his belief, including a poem that he called the Thalia. Most of these writings were destroyed as being heretical, though portions of the Thalia and a few other Arian texts survive. The Roman emperor Constantine the Great , desiring the restoration of peace and unity to the Church, publicly called upon Arius and Alexander to settle their dispute; however, the issue was such that no genuine compromise was possible. As the debate continued to rage between supporters of each man, the emperor finally decided to call a great council of all Church bishops to resolve the dilemma. This First Ecumenical Council , held at Nicea in 325, was led in its teachings by Athanasius, at this time a mere deacon in the Alexandrian church. The council condemned Arianism and maintained that Christ was "God from God, Light from Light, Very God from Very God, begotten not made (not created), and One in essence with the Father." (homoousios/ομοούσιος "of the same essence" rather than Arius' heretical homoiousios/ομοιούσιος "of a similar essence"). It then incorporated these words into the first version of the Nicene Creed.