5 They think also that they have a compendious refutation of Prophets, Evangelists and Apostles alike, in their assertion that the Son was born within time. They pronounce us illogical for saying that the Son has existed from everlasting; and, since they reject the possibility of His eternity, they are forced to believe that He was born at a point in time. For if He has not always existed, there was a time when He was not; and if there be a time when He was not, time was anterior to Him. He who has not existed everlastingly began to exist within time, while He Who is free from the limits of time is necessarily eternal. The reason they give for their rejection of the eternity of the Son is that His everlasting existence contradicts the faith in His birth; as though by confessing that He has existed eternally, we made His birth impossible.
Source: On the Trinity (New Advent)