Answer to the question he is always asking, Can He who is be begotten?
Since, then, He who is with the Father, in some inconceivable category, before the ages admits not of a 'sometime,' He exists by generation indeed, but nevertheless He never begins to exist. His life is neither in time, nor in place. But when we take away these and all suchlike ideas in contemplating the subsistence of the Son, there is only one thing that we can even think of as before Him— i.e. the Father. But the Only-begotten, as He Himself has told us, is in the Father, and so, from His nature, is not open to the supposition that He ever existed not. If indeed the Father ever was not, the eternity of the Son must be cancelled retrospectively in consequence of this nothingness of the Father: but if the Father is always, how can the Son ever be non-existent, when He cannot be thought of at all by Himself apart from the Father, but is always implied silently in the name Father. This name in fact conveys the two Persons equally; the idea of the Son is inevitably suggested by that word. When was it, then, that the Son was not? In what category shall we detect His non-existence? In place? There is none. In time? Our Lord was before all times; and if so, when was He not? And if He was in the Father, in what place was He not? Tell us that, you who are so practised in seeing things out of sight. What kind of interval have your cogitations given a shape to? What vacancy in the Son, be it of substance or of conception, have you been able to think of, which shows the Father's life, when drawn out in parallel, as surpassing that of the Only-begotten? Why, even of men we cannot say absolutely that any one was not, and then was born. Levi, many generations before his own birth in the flesh, was tithed by Melchisedech; so the Apostle says, “Levi also, who receives tithes, payed tithes (in Abraham),” adding the proof, “for he was yet in the loins of his father, when” Abraham met the priest of the Most High. If, then, a man in a certain sense is not, and is then born, having existed beforehand by virtue of kinship of substance in his progenitor, according to an Apostle's testimony, how as to the Divine life do they dare to utter the thought that He was not, and then was begotten? For He 'is in the Father,' as our Lord has told us; “I am in the Father, and the Father in Me,” each of course being in the other in two different senses; the Son being in the Father as the beauty of the image is to be found in the form from which it has been outlined; and the Father in the Son, as that original beauty is to be found in the image of itself. Now in all hand-made images the interval of time is a point of separation between the model and that to which it lends its form; but there the one cannot be separated from the other, neither the “express image” from the “Person,” to use the Apostle's words, nor the “brightness” from the “glory” of God, nor the representation from the goodness; but when once thought has grasped one of these, it has admitted the associated Verity as well. “Being,” he says (not becoming), “the brightness of His glory;” so that clearly we may rid ourselves for ever of the blasphemy which lurks in either of those two conceptions; viz., that the Only-begotten can be thought of as Ungenerate (for he says “the brightness of His glory,” the brightness coming from the glory, and not, reversely, the glory from the brightness); or that He ever began to be. For the word “being” is a witness that interprets to us the Son's continuity and eternity and superiority to all marks of time.
What occasion, then, had our foes for proposing for the damage of our Faith that trifling question, which they think unanswerable and, so, a proving of their own doctrine, and which they are continually asking, namely, 'whether One who is can be generated.' We may boldly answer them at once, that He who is in the Ungenerate was generated from Him, and does derive His source from Him. 'I live by the Father:' but it is impossible to name the 'when' of His beginning. When there is no intermediate matter, or idea, or interval of time, to separate the being of the Son from the Father, no symbol can be thought of, either, by which the Only-begotten can be unlinked from the Father's life, and shown to proceed from some special source of His own. If, then, there is no other principle that guides the Son's life, if there is nothing that a devout mind can contemplate before (but not divided from) the subsistence of the Son, but the Father only; and if the Father is without beginning or generation, as even our adversaries admit, how can He who can be contemplated only within the Father, who is without beginning, admit Himself of a beginning?
What harm, too, does our Faith suffer from our admitting those expressions of our opponents which they bring forward against us as absurd, when they ask 'whether He which is can be begotten?' We do not assert that this can be so in the sense in which Nicodemus put his offensive question, wherein he thought it impossible that one who was in existence could come to a second birth: but we assert that, having His existence attached to an Existence which is always and is without beginning, and accompanying every investigator into the antiquities of time, and forestalling the curiosity of thought as it advances into the world beyond, and intimately blended as He is with all our conceptions of the Father, He has no beginning of His existence any more than He is Ungenerate: but He was both begotten and was, evincing on the score of causation generation from the Father, but by virtue of His everlasting life repelling any moment of non-existence.
Source: Against Eunomius (New Advent)