It's amazing how important one word can be. At the Council of Nicea, that word was homoousios. Seven centuries later one word, filioque, would create the biggest church split in history.
What a famous word homoousios would become. I heard one pastor, who shares my avid interest in the Pre-Nicene church, say that in Alexandria there were bar fights over the word homoousios.
That's quite likely to be true, what with Arius having jingles sung in the marketplace.
Homoousios means "one substance" or "same substance." This is really important.
Please bear with me while I explain why.
To the Greeks, substance was everything. All matter, to them, was made up of earth, wind, fire, or air. God, however, was none of those things. Everything—no, really: everything— else was.
Even angels and the spirits of men were matter by Greek thinking. In a sense, there are only two substances in the universe. One is God—whatever the substance is that constitutes God—and the other is matter. Athenagoras, a Christian apologist writing in A.D. 168, tells us:
The question being asked at the Council of Nicea was …
That may seem strange to us modern Christians who are not used to the question at all, but at the Council of Nicea, it was everything (thus the two references to God's substance in the Nicene Creed).
Athenagoras goes on to answer that question immediately:
Justin Martyr, a couple decades earlier, adds his testimony to that of Athenagoras:
Many more such quotes could be found. The unity of substance between the Father and the Son, and the distinction between the Son and matter, from which all else was made, is often discussed by Pre-Nicene Christian writers.
All of this applied to Arius' theology at the Council of Nicea. If the Son had a beginning, as Arius was asserting, then he must be made of matter. After all, the substance of God can have no beginning.
On the other hand, if Christ is of God's substance, then he always existed.
It seems Arius was confused by the fact that the Church taught that Jesus was begotten of the Father, not just here on earth as a man, but in the beginning as the Son of God. Proverbs 8:22, in the Greek Septuagint version used by the early churches, says, "The Lord made me the beginning of his ways for his works." The early churches universally understood this to refer to the begetting of Christ from the Father in eternity past.
Arius mistakenly thought and argued that the begetting of the Son constitutes a beginning. He could not have existed before then.
The Church, however, taught that before Jesus was begotten by the Father, he already existed inside the Father as the Word, Wisdom, and Reason of the Father. The Father had always had Logos inside himself, so Jesus had always existed inside of him. (This is described thoroughly in Tertullian's Against Praxeas, ch. 5.)
Not only is this what they taught, but they claimed to have received it from the apostles. Thus, the idea that the Son has always existed qualified as apostolic tradition, which carried as much—or perhaps almost as much—authority as Scripture (Irenaeus, ibid. III:2:2). This is the tradition that Arius was disagreeing with, and this is the reason that Constantine wanted homoousios inserted in the Creed of Nicea.