Several ways of controverting his quibbling syllogisms
But let us look to the thought in it below the words. 'If God is Ungenerate because He has begotten a Son, He was not Ungenerate before He begot Him.' The answer to that is plain; it consists in the simple statement of the Truth that 'the word Father means both the having begotten a Son, and also that the Begetter is not to be thought of as Himself coming from any cause.' If you look at the effect, the Person of the Son is revealed in the word Father; if you look for a previous Cause, the absence of any beginning in the Begetter is shown by that word. In saying that 'Before He begot a Son, the Almighty was not Ungenerate,' this pamphleteer lays himself open to a double charge; i.e. of misrepresentation of us, and of insult to the Faith. He attacks, as if there was no mistake about it, something which our Teacher never said, neither do we now assert, viz., that the Almighty became in process of time a Father, having been something else before. Moreover in ridiculing the absurdity of this fancied doctrine of ours, he proclaims his own wildness as to doctrine. Assuming that the Almighty was once something else, and then by an advance became entitled to be called Father, he would have it that before this He was not Ungenerate either, since Ungeneracy is implied in the idea of Father. The folly of this hardly needs to be pointed out; it will be abundantly clear to anyone who reflects. If the Almighty was something else before He became Father, what will the champions of this theory say, if they were asked in what state they propose to contemplate Him? What name are they going to give Him in that stage of existence; child, infant, babe, or youth? Will they blush at such flagrant absurdity, and say nothing like that, and concede that He was perfect from the first? Then how can He be perfect, while as yet unable to become Father? Or will they not deprive Him of this power, but say only that it was not fitting that there should be Fatherhood simultaneously with His existence. But if it was not good nor fitting that He should be from the very beginning Father of such a Son, how did He go on to acquire that which was not good?
But, as it is, it is good and fitting to God's majesty that He should become Father of such a Son. So they will make out that at the beginning He had no share in this good thing, and as long as He did not have this Son they must assert (may God forgive me for saying it!) that He had no Wisdom, nor Power, nor Truth, nor any of the other glories which from various points of view the Only-begotten Son is and is called.
But let all this fall on the heads of those who started it. We will return whence we digressed. He says, “if God is Father because of having begotten a Son, and if Father means the being Ungenerate, then God was not this last, before He begot.” Now if he could speak here as it is customary to speak about human life, where it is inconceivable that any should acquire possession of many accomplishments all at once, instead of winning each of the objects sought after in a certain order and sequence of time— if I say we could reason like that in the case of the Almighty, so that we could say He possessed His Ungeneracy at one time, and after that acquired His power, and then His imperishability, and then His Wisdom, and advancing so became Father, and after that Just and then Everlasting, and so came into all that enters into the philosophical conception of Him, in a certain sequence— then it would not be so manifestly absurd to think that one of His names has precedence of another name, and to talk of His being first Ungenerate, and after that having become Father.
As it is, however, no one is so earth-bound in imagination, so uninitiated in the sublimities of our Faith, as to fail, when once he has apprehended the Cause of the universe, to embrace in one collective and compact whole all the attributes which piety can give to God; and to conceive instead of a primal and a later attribute, and of another in between, supervening in a certain sequence. It is not possible, in fact, to traverse in thought one among those attributes and then reach another, be it a reality or a conception, which is to transcend the first in antiquity. Every name of God, every sublime conception of Him, every utterance or idea that harmonizes with our general ideas with regard to Him, is linked in closest union with its fellow; all such conceptions are massed together in our understanding into one collective and compact whole namely, His Fatherhood, and Ungeneracy, and Power, and Imperishability, and Goodness, and Authority, and everything else. You cannot take one of these and separate it in thought from the rest by any interval of time, as if it preceded or followed something else; no sublime or adorable attribute in Him can be discovered, which is not simultaneously expressed in His everlastingness. Just, then, as we cannot say that God was ever not good, or powerful, or imperishable, or immortal, in the same way it is a blasphemy not to attribute to Him Fatherhood always, and to say that that came later. He Who is truly Father is always Father; if eternity was not included in this confession, and if a foolishly preconceived idea curtailed and checked retrospectively our conception of the Father, true Fatherhood could no longer be properly predicated of Him, because that preconceived idea about the Son would cancel the continuity and eternity of His Fatherhood. How could that which He is now called be thought of something which came into existence subsequent to these other attributes? If being first Ungenerate He then became Father, and received that name, He was not always altogether what He is now called. But that which the God now existing is He always is; He does not become worse or better by any addition, He does not become altered by taking something from another source. He is always identical with Himself. If, then, He was not Father at first, He was not Father afterwards. But if He is confessed to be Father (now), I will recur to the same argument, that, if He is so now, He always was so; and that if He always was, He always will be. The Father therefore is always Father; and seeing that the Son must always be thought of along with the Father (for the title of father cannot be justified unless there is a son to make it true), all that we contemplate in the Father is to be observed also in the Son. “All that the Father has is the Son's; and all that is the Son's the Father has.” The words are, 'The Father has that which is the Son's,' and so a carping critic will have no authority for finding in the contents of the word “all” the ungeneracy of the Son, when it is said that the Son has all that the Father has, nor on the other hand the generation of the Father, when all that is the Son's is to be observed in the Father. For the Son has all the things of the Father; but He is not Father: and again, all the things of the Son are to be observed in the Father, but He is not a Son.
Source: Against Eunomius (New Advent)