28 We may grant that for anything to be born before times eternal is not the way of human nature, nor a matter which we can understand; and yet in this we believe God's declarations about Himself. How then does the infidelity of our own day assert, according to the conceptions of human intelligence, that that had no existence before it was born, which the Apostolic faith tells us was, in some manner inconceivable to the human understanding, always born, or in other words existed before times eternal?
For what is born before time is always born; since that which exists before time eternal, always exists. But what has always been born, cannot at any time have had no existence; since non-existence at a given time is directly contrary to eternity of existence. Moreover, existing always excludes the idea of not having existed always. And the idea of not having existed always being excluded by the postulate that He has always been born, we cannot conceive the supposition that He did not exist before He was born.
For it is obvious that He Who was born before times eternal, has always been born, although we can form no positive conception of anything having been born before all time. For if we must confess (as is clearly necessary) that He has been born before every creature, whether invisible or corporeal, and before all ages and times eternal, and before all perception, Who always exists through the very fact that He has been so born—then by no manner of thought can it be conceived that before He was born, He did not exist; since He Who has been born before times eternal, is prior to all thought, and we can never think that once He did not exist, when we have to confess that He always exists.
Source: On the Trinity (New Advent)