That the Son is Eternal and Increate. These attributes, being the points in dispute, are first proved by direct texts of Scripture. Concerning the 'eternal power' of God in Romans 1:20, which is shown to mean the Son. Remarks on the Arian formula, 'Once the Son was not,' its supporters not daring to speak of 'a time when the Son was not.'
11 At his suggestion then you have maintained and you think, that 'there was once when the Son was not;' this is the first cloke of your views of doctrine which has to be stripped off. Say then what was once when the Son was not, O slanderous and irreligious men? If you say the Father, your blasphemy is but greater; for it is impious to say that He was 'once,' or to signify Him by the word 'once.' For He is ever, and is now, and as the Son is, so is He, and is Himself He that is, and Father of the Son. But if you say that the Son was once, when He Himself was not, the answer is foolish and unmeaning. For how could He both be and not be? In this difficulty, you can but answer, that there was a time when the Word was not; for your very adverb 'once' naturally signifies this. And your other, 'The Son was not before His generation,' is equivalent to saying, 'There was once when He was not,' for both the one and the other signify that there is a time before the Word. Whence then this your discovery? Why do you, as 'the heathen, rage, and imagine vain phrases against the Lord and against His Christ.' for no holy Scripture has used such language of the Saviour, but rather 'always' and 'eternal' and 'coexistent always with the Father.' For, 'In the beginning was the Word, and the Word was with God, and the Word was God.' And in the Apocalypse he thus speaks; 'Who is and who was and who is to come.' Now who can rob 'who is' and 'who was' of eternity? This too in confutation of the Jews has Paul written in his Epistle to the Romans, 'Of whom as concerning the flesh is Christ, who is over all, God blessed for ever;' while silencing the Greeks, he has said, 'The visible things of Him from the creation of the world are clearly seen, being understood by the things that are made, even His eternal Power and Godhead;' and what the Power of God is, he teaches us elsewhere himself, 'Christ the Power of God and the Wisdom of God. ' Surely in these words he does not designate the Father, as you often whisper one to another, affirming that the Father is 'His eternal power.' This is not so; for he says not, 'God Himself is the power,' but 'His is the power.' Very plain is it to all that 'His' is not 'He;' yet not something alien but rather proper to Him. Study too the context and 'turn to the Lord.' now 'the Lord is that Spirit;'and you will see that it is the Son who is signified.
12. For after making mention of the creation, he naturally speaks of the Framer's Power as seen in it, which Power, I say, is the Word of God, by whom all things have been made. If indeed the creation is sufficient of itself alone, without the Son, to make God known, see that you fall not, from thinking that without the Son it has come to be. But if through the Son it has come to be, and 'in Him all things consist,' it must follow that he who contemplates the creation rightly, is contemplating also the Word who framed it, and through Him begins to apprehend the Father. And if, as the Saviour also says, 'No one knows the Father, save the Son, and he to whom the Son shall reveal Him,' and if on Philip's asking, 'Show us the Father,' He said not, 'Behold the creation,' but, 'He that has seen Me, has seen the Father,' reasonably does Paul—while accusing the Greeks of contemplating the harmony and order of the creation without reflecting on the Framing Word within it (for the creatures witness to their own Framer) so as through the creation to apprehend the true God, and abandon their worship of it—reasonably has he said, 'His Eternal Power and Godhead,' thereby signifying the Son. And where the sacred writers say, 'Who exists before the ages,' and 'By whom He made the ages,' they thereby as clearly preach the eternal and everlasting being of the Son, even while they are designating God Himself. Thus, if Isaiah says, 'The Everlasting God, the Creator of the ends of the earth;' and Susanna said, 'O Everlasting God;' and Baruch wrote, 'I will cry unto the Everlasting in my days,' and shortly after, 'My hope is in the Everlasting, that He will save you, and joy has come unto me from the Holy One;' yet forasmuch as the Apostle, writing to the Hebrews, says, 'Who being the radiance of His glory and the Expression of His Person;' and David too in the eighty-ninth Psalm, 'And the brightness of the Lord be upon us,' and, 'In Your Light shall we see Light,' who has so little sense as to doubt of the eternity of the Son? For when did man see light without the brightness of its radiance, that he may say of the Son, 'There was once, when He was not,' or 'Before His generation He was not.' And the words addressed to the Son in the hundred and forty-fourth Psalm, 'Your kingdom is a kingdom of all ages,' forbid any one to imagine any interval at all in which the Word did not exist. For if every interval in the ages is measured, and of all the ages the Word is King and Maker, therefore, whereas no interval at all exists prior to Him, it were madness to say, 'There was once when the Everlasting was not,' and 'From nothing is the Son.' And whereas the Lord Himself says, 'I am the Truth,' not 'I became the Truth.' but always, 'I am—I am the Shepherd,— I am the Light,'— and again, 'Call Me not, Lord and Master? And you call Me well, for so I am,' who, hearing such language from God, and the Wisdom, and Word of the Father, speaking of Himself, will any longer hesitate about the truth, and not immediately believe that in the phrase 'I am,' is signified that the Son is eternal and without beginning?
Source: Four Discourses Against the Arians (New Advent)