54 His birth is before times eternal. If anything exist which precedes eternity, it will be something which, when eternity is comprehended, still eludes comprehension. And this something is Yours, and is Your Only-begotten; no portion, nor extension, nor any empty name devised to suit some theory of Your mode of action. He is the Son, a Son born of You, God the Father, Himself true God, begotten by You in the unity of Your nature, and meet to be acknowledged after You, and yet with You, since You are the eternal Author of His eternal origin. For since He is from You, He is second to You; yet since He is Yours, You are not to be separated from Him. For we must never assert that Thou once existed without Your Son, lest we should be reproaching You either with imperfection, as then unable to generate, or with superfluousness after the generation. And so the exact meaning for us of the eternal generation is that we know You to be the eternal Father of Your Only-begotten Son, Who was born of You before times eternal.
Source: On the Trinity (New Advent)