Two senses of the word Son, 1. adoptive; 2. essential; attempts of Ariansto find a third meaning between these; e.g. that our Lord only was created immediately by God (Asterius's view), or that our Lord alone partakes the Father. The second and true sense; God begets as He makes, really; though His creation and generation are not like man's; His generation independent of time; generation implies an internal, and therefore an eternal, act in God; explanation of Proverbs 8:22
12 The case being thus, let who will among them consider the matter, so that one may abash them by the following question; Is it right to say that what is God's offspring and proper to Him is out of nothing? Or is it reasonable in the very idea, that what is from God has accrued to Him, that a man should dare to say that the Son is not always? For in this again the generation of the Son exceeds and transcends the thoughts of man, that we become fathers of our own children in time, since we ourselves first were not and then came into being; but God, in that He ever is, is ever Father of the Son. And the origination of mankind is brought home to us from things that are parallel; but, since 'no one knows the Son but the Father, and no one knows the Father but the Son, and he to whomsoever the Son will reveal Him,' therefore the sacred writers to whom the Son has revealed Him, have given us a certain image from things visible, saying, 'Who is the brightness of His glory, and the Expression of His Person;' and again, 'For with You is the well of life, and in Your light shall we see light;' and when the Word chides Israel, He says, 'You have forsaken the Fountain of wisdom;' and this Fountain it is which says, 'They have forsaken Me the Fountain of living waters. ' And mean indeed and very dim is the illustration compared with what we desiderate; but yet it is possible from it to understand something above man's nature, instead of thinking the Son's generation to be on a level with ours. For who can even imagine that the radiance of light ever was not, so that he should dare to say that the Son was not always, or that the Son was not before His generation? Or who is capable of separating the radiance from the sun, or to conceive of the fountain as ever void of life, that he should madly say, 'The Son is from nothing,' who says, 'I am the life,' or 'alien to the Father's essence,' who, says, 'He that has seen Me, has seen the Father?' for the sacred writers wishing us thus to understand, have given these illustrations; and it is unseemly and most irreligious, when Scripture contains such images, to form ideas concerning our Lord from others which are neither in Scripture, nor have any religious bearing.
13. Therefore let them tell us, from what teacher or by what tradition they derived these notions concerning the Saviour? “We have read,” they will say, “in the Proverbs, 'The Lord created me a beginning of His ways unto His works;'” this Eusebius and his fellows used to insist on, and you write me word, that the present men also, though overthrown and confuted by an abundance of arguments, still were putting about in every quarter this passage, and saying that the Son was one of the creatures, and reckoning Him with things originated. But they seem to me to have a wrong understanding of this passage also; for it has a religious and very orthodox sense, which had they understood, they would not have blasphemed the Lord of glory. For on comparing what has been above stated with this passage, they will find a great difference between them. For what man of right understanding does not perceive, that what are created and made are external to the maker; but the Son, as the foregoing argument has shown, exists not externally, but from the Father who begot Him? For man too both builds a house and begets a son, and no one would reverse things, and say that the house or the ship were begotten by the builder, but the son was created and made by him; nor again that the house was an image of the maker, but the son unlike him who begot him; but rather he will confess that the son is an image of the father, but the house a work of art, unless his mind be disordered, and he beside himself. Plainly, divine Scripture, which knows better than any the nature of everything, says through Moses, of the creatures, 'In the beginning God created the heaven and the earth;' but of the Son it introduces not another, but the Father Himself saying, 'I have begotten You from the womb before the morning star;' and again, 'You are My Son, this day have I begotten You. ' And the Lord says of Himself in the Proverbs, 'Before all the hills He begets me;' and concerning things originated and created John speaks, 'All things were made by Him;' but preaching of the Lord, he says, 'The Only-begotten Son, who is in the bosom of the Father, He declared Him. ' If then son, therefore not creature; if creature, not son; for great is the difference between them, and son and creature cannot be the same, unless His essence be considered to be at once from God, and external to God.
14. 'Has then the passage no meaning?' for this, like a swarm of gnats, they are droning about us. No surely, it is not without meaning, but has a very apposite one; for it is true to say that the Son was created too, but this took place when He became man; for creation belongs to man. And any one may find this sense duly given in the divine oracles, who, instead of accounting their study a secondary matter, investigates the time and characters, and the object, and thus studies and ponders what he reads. Now as to the season spoken of, he will find for certain that, whereas the Lord always is, at length in fullness of the ages He became man; and whereas He is Son of God, He became Son of man also. And as to the object he will understand, that, wishing to annul our death, He took on Himself a body from the Virgin Mary; that by offering this unto the Father a sacrifice for all, He might deliver us all, who by fear of death were all our life through subject to bondage. And as to the character, it is indeed the Saviour's, but is said of Him when He took a body and said, 'The Lord created me a beginning of His ways unto His works.' For as it properly belongs to God's Son to be everlasting. and in the Father's bosom, so on His becoming man, the words befitted Him, 'The Lord created me.' For then it is said of Him, as also that He hungered, and thirsted, and asked where Lazarus lay, and suffered, and rose again. And as, when we hear of Him as Lord and God and true Light, we understand Him as being from the Father, so on hearing, 'The Lord created,' and 'Servant,' and 'He suffered,' we shall justly ascribe this, not to the Godhead, for it is irrelevant, but we must interpret it by that flesh which He bore for our sakes: for to it these things are proper, and this flesh was none other's than the Word's. And if we wish to know the object attained by this, we shall find it to be as follows: that the Word was made flesh in order to offer up this body for all, and that we partaking of His Spirit, might be deified, a gift which we could not otherwise have gained than by His clothing Himself in our created body, for hence we derive our name of “men of God” and “men in Christ.” But as we, by receiving the Spirit, do not lose our own proper substance, so the Lord, when made man for us, and bearing a body, was no less God; for He was not lessened by the envelopment of the body, but rather deified it and rendered it immortal.
Source: De Decretis (New Advent)