23 But does any one doubt that all human beings that have been born, at one time were not? It is, however, one thing to be born of some one who once was not, and another to be born of One Who always is. For every state of infancy, since previously it had no existence, began from some point of time. And this again, growing up into childhood, still later urges on youth to fatherhood. Yet the man was not always a father, for he advanced to youth through boyhood, and to boyhood through original infancy.
Therefore he who was not always a father, also did not always beget: but where the Father is eternal, the Son also is eternal. And so if you hold, whether by argument or by instinct, that God, in the mystery of our knowledge of Whom one property is that He is Father, was not always the Father of the begotten Son, you hold also, as a matter of understanding and of knowledge, that the Son, Who was begotten, did not always exist. But if the property of fatherhood be co-eternal with the Father, then necessarily also the property of sonship must be co-eternal with the Son. And how will it square with our language or our understanding to maintain that He was not before He was born, Whose property it is that He always was what He has been born.
Source: On the Trinity (New Advent)