Athanasius Defense of the Nicene Definition
In his letter defending the Nicene Definition, Athanasius concerns himself with several charges laid against the Nicene Council.
The term begotten is the springboard from which the Arians viewed Christ not as the Creator, but rather as a creature of the Father. The first begotten Son, after being created, became the means by which the Father created all other things. Hence, this creature cannot be the same essence as the Father, and as such is not True God. Athanasius responds with several arguments. Two considerations follow.
Man’s Contingency & God’s Necessity
First, he considers man’s contingency and God’s necessity and relates it to our natures. He points out that in order for man to create there must already exist material, whereas for God to create, he only has to speak the word ex-nihilo. He continues and points out that man’s generation is in one way, and the Son’s from the Father is another. Man’s offspring by nature is compounded in begetting children, but God who by nature is uncompounded, is Father of the One Only Son. That is to say, that the Son is eternally generated from the Father, for in that God ever is, He is ever the Father of the Son. Athanasius follows this argument and supports it with Scriptures (Mt.9: 27; Heb.1: 3; Ps.36: 9; Jn.14: 9).
Confronting the Arian’s Misinterpretation of Scripture
Second, Athanasius is aware of how the Arians misinterpret Scripture. They argue for the creation of Christ from Proverbs 8:22 “The Lord created me a beginning of His ways unto His works” and his response is that it does refer to the Son in his humanity, for creation belongs to man. Moreover, just as we do not lose our proper substance when we receive the Spirit, so Christ did not lose his substance of deity when he became man, but rather he deified and rendered it immortal. Athansius then continues explaining the Catholic sense of the word Son, and asserts that his name implies eternal.
The Phrases “From the Essence” and “One in Essence.”
The Arian’s complain that the terms, “Of the essence” and “One in essence” is not Scriptural. Athanasius quickly exposes their hypocrisy by asking “why do they [Arians] use phrases like ‘He was not before His generation,’ and ‘once he was not,’ and ‘out of nothing,’ and ‘pre-existence,’ which are clearly not Scriptural.” He then indicts them of making up fables and mocking the Lord. He then explains the reason for the usage of these phrases and their meaning.
The phrase ‘from God’ was understood by the Arians to mean that Christ, like men, is the offspring of God. To combat the heterodoxy, they chose the phrase ‘from the essence of God’ so that the Son would not be seen as a creature, but rather as the Word, which is from the Father, who is the originator of all things, truly from God. This phrase was installed to prevent any deception from the Arians.
“One in Essence”
The phrase ‘one in essence’ describes the indivisibility of the Father and the Son, and it was written by the Council to defeat the twisted heretics, and to show that the Word is not a mutable creature, but rather the Creator of all creatures, of all things. Moreover, the Council anathematized the Arian doctrine, and Athanasius then challenges the Arians to refute the Council’s position. If they can, then “anathematize” the anathema of the Council. If there are those who think the phrase is strange, Athanasius affirms that this is so because they are not understood with the intended meaning of the Council. Hence, Athanasius is essentially telling the Arians to “put up or shut up.”
Athanasius continues and sites several authorities that agree with the Council on the phraseology, and finishes the letter by grappling with the unscriptural term unoriginate that the Arian’s borrowed from the Greeks.
 Athanasius, “Defense of the Nicene Definition,” The Nicene and Post-Nicene Fathers, Second Series,
Volume IV, Pp.150-172, (T & T Clark Edinburgh, Wm. B. Eerdmans Publishing Company, Reprinted in 1996)