One of the most helpful books for understanding the usage of extra-Biblical words in the Nicene Creed is “A Defense of the Nicene Definition” by Athanasius (who was at the Nicene Council). This post can be seen as a follow up to this post and this post. In those posts, I related how thoroughly Biblical the language of the Nicene Creed was over against those who would claim that the Creed is full of philosophical notions and language. However, Athanasius had much more insight into why certain things were written.
I would like to begin with one of his later statements that it was the Arians who “have to blame themselves, and no one else in this matter, for it was they who set the example, beginning their war against God with statements not in Scripture”. And still later in this work, Athanasius stated that these were some of the specific phrases not found in Scripture which the Arians were using:
But those who say, ‘Once He was not,’ and ‘Before His generation He was not,’ and ‘He came into being from nothing,’ or those who pretend that the Son of God is ‘Of other subsistence or substance,’ or ‘created,’ or ‘alterable,’ or ‘mutable,’ the Catholic Church anathematises.
It is with these above “impious phrases of the Arians” in mind that we can go to an earlier section in his work. Note below that Athanasius tells us that the Bishops at Nicea “were forced thereupon to express more distinctly the sense of the words from God.” Ironically, many today would claim that the Arians were “Biblicists” insisting on just using the words of Scripture and being led into error because of that. However, it was clear that Athanasius and the Nicene Council wished that they could have stuck closer to scripture. Their departure from biblical language was to clarify what they saw as the “sense of the words from God.”
“The Council wishing to condemn the impious phrases of the Arians, and to use instead the received terms of Scripture, namely, that the Son is not from nothing, but from God, and is the Word and Wisdom and not a creature or work, but the proper Offspring from the Father…. But the Fathers, perceiving their craft and the cunning of their impiety, were forced thereupon to express more distinctly the sense of the words from God. Accordingly, they wrote “from the substance of God,” in order that from God might not be considered common and equal in the Son and in things which are made, but that all others might be acknowledged as creatures, and the Word alone as from the Father. For though all things be said to be from God, yet this is not in the sense in which the Son is from Him;”
Shortly after this, Athanasius wrote the following.
on this account did the Holy Council declare expressly that He was of the substance of the Father, that we might believe the Word to be other in nature than things which have a beginning, as being alone truly from God; and that no subterfuge should be left open to the impious. This then was the reason why the Council wrote “Of the substance.”… the Bishops said that the Word must be described as the True Power and image of the Father as the exact Likeness of the Father in all things, and as unalterable, and as always, and as in Him without division; (for never was the Word not in being, but He was always existing everlastingly with the Father, as the radiance of light),
And he then clarified it further with this statement. “Consubstantial” was used as a defining term to show that the Word (Son) was not among “created things”. According to Athanasius, by the Council saying “of the substance of God” or “Consubstantial” was the same as reading in Scripture that “the Word was of God”.
therefore the Council, as understanding this, suitably wrote “Consubstantial,” that they might both defeat the perverseness of the heretics, and show that the Word was other than created things. For they considered it the same thing to say that the Word was of God and “of the substance of God,” since the word “God,” as I have already said, signifies nothing but the substance or essence of Him who is.
I would highly recommend that you read through “A Defense of the Nicene Definition” by Athanasius (it is only around 40 pages). It is a helpful behind-the-scenes look at some of the decisions taken by the Bishops given the great duty to put together a brief statement of faith in order to combat the assault on the Trinity by the Arians. Athanasius tells us that they would like to have stuck even more closely to Scripture (the words from God), but in order to defend against the phrases being used by the Arians (phrases to describe the Son which were not in Scripture) they had to make some concessions. Even in doing so, the words used in the Nicene Creed were easily seen to be a summary of what the Bishops understood from reading Scripture.