18 Now I am not ignorant that most of those, whose mind being dulled by impiety does not accept the mystery of God, or who through the strong influence of a hostile spirit are ready to manifest, under the cover of reverence, a mad passion for disparaging God, are wont to make strange assertions in the ears of simple-minded men. They assert that since we say that the Son always has been, and that He never has been anything which He has not always been, we are therefore declaring that He is without birth, inasmuch as He always has been; since, according to the workings of human reason, that which always has existed cannot possibly have been born: since (so they urge) the cause of a thing being born, is that something, which was not, may come into existence, while the coming into existence of something which was not, means nothing else, according to the judgment of common sense, than its being born.
They may add those arguments, subtle enough and pleasant to hear—“If He was born, He began to be; at the time when He began to be, He was not: and when He was not, it cannot be that He was.” By such proofs let them maintain that it is the language of reasonable piety to say, “He was not before He was born: because in order that He might come to be, One Who was not, not One Who was, was born. Nor did He Who was, require a birth, although He Who was not was born, to the end that He might come to be.”
Source: On the Trinity (New Advent)