25 Therefore He was, and He is, since He is from Him Who always is what He is. But to be from Him, that is to say, to be from the Father, is birth. Moreover, to be always from Him, Who always is, is eternity; but this eternity is derived not from Himself, but from the Eternal. And from the Eternal nothing can spring but what is eternal: for if the Offspring is not eternal, then neither is the Father, Who is the source of generation, eternal. Now since it is the special characteristic of His being that His Father always exists, and that He is always His Son, and since eternity is expressed in the name He that is, therefore, since He possesses absolute being, He possesses also eternal being.
Moreover, no one doubts that generation implies birth, and that birth points to one existing from that time forth, and not to one who does not continue. Furthermore, there can be no doubt that no one who already was in existence could be born. For no cause of birth can accrue to Him, Who of Himself continues eternal. But God Only-begotten, Who is the Wisdom of God, and the Power and the Word of God, since He was born, bears witness to the Father as the source of His being. Since He was born of One, Who eternally exists, He was not born of nothing.
Since He was born before times eternal, His birth must necessarily be prior to all thought. There is no room for the verbal quibble, “He was not, before He was born.” For if He is within the range of our thought, in the sense that He was not before He was born, then both our thought and time are prior to His birth; since everything which once was not, is within the compass of thought and time, by the very meaning of the assertion that it once was not, which separates off, within time, a period when it did not exist. But He is from the Eternal, and yet has always been; He is not ingenerate, yet never was non-existent; since to have always been transcends time, and to have been born is birth.
Source: On the Trinity (New Advent)