31 Again, this same fact excludes the possibility of saying that He existed before He was born; because He Who transcends perception transcends it in every respect. For if the notion of being born, though always existing, transcends thought, it is equally impossible that the notion that He did not exist before He was born should be a subject of thought. And so, since we must confess that to have been always born means for us nothing beyond the fact of birth, the question whether He did or did not exist before He was born cannot be determined under our conditions of thought; since this one fact that He was born before times eternal ever eludes the grasp of our thought.
So He was born and yet has always existed; He Who does not allow anything else to be understood or said about Him than that He was born. For since He is prior to time itself within which thought exists (since time eternal is previous to thought), He debars thought from determining concerning Him, whether He was or was not before He was born; since existence before birth is incompatible with the idea of birth, and previous non-existence involves the idea of time. Therefore, while the infinity of times eternal is fatal to any explanation involving the idea of time— that is to say, to the notion that He did not exist; His birth equally forbids any that is inconsistent with it—that is to say, the notion that He existed before He was born.
For if the question of His existence or His non-existence can be determined under our conditions of thought, then the birth itself must be after time; for He Who does not always exist must, of necessity, have begun to be after some given point of time.
Source: On the Trinity (New Advent)