21 Accordingly let misbelief abandon its efforts; let it not think, because it does not understand, that we deny a truth which, in fact, we alone rightly understand and believe. For while we declare in so many words that He was born, nevertheless we do not assert that He was ever not born. For it is not the same thing to be not born and to be born: since the latter term expresses origin derived from some other, the former origin derived from none. And it is one thing to exist always, as the Eternal, without any source of being, and another to be co-eternal with a Father, having Him for the Source of being.
For where a father is the source of being, there also is birth; and further, where the Source of being is eternal, the birth also is eternal: for since birth comes from the source of being, birth which comes from an eternal Source of being must be eternal. Now everything which always exists, is also eternal. But nevertheless, not everything which is eternal is also not born; since that which is born from eternity has eternally the character of having been born; but that which is not born is ingenerate as well as eternal.
But if that which has been born from the Eternal is not born eternal, it will follow that the Father also is not an eternal Source of being. Therefore if any measure of eternity is wanting to Him Who has been born of the eternal Father, clearly the very same measure is wanting to the Author of His being; since what belongs in an infinite degree to Him Who begets, belongs in an infinite degree to Him also Who is born. For neither reason nor intelligence allows of any interval between the birth of God the Son and the generation by God the Father; since the generation consists in the birth, and the birth in the generation.
Thus each of these events coincides exactly with the other; neither took place unless both took place. Therefore that which owes its existence to both these events cannot be eternal unless they both are eternal; since neither of the two correlatives, apart from the other, has any reality, because it is impossible for one to exist without the other.
Source: On the Trinity (New Advent)