17 But for God Only-begotten, Who is preceded by no antecedent time, the possibility is excluded that at some time He was not, since that “some time” thus becomes prior to Him; and again, the assertion that He was not involves the notion of time: whence time will not begin to be after Him, but He Himself will begin to be after time, and, inasmuch as He was not before He was born, the very period when He was not will take precedence of Him. Further, He Who is born from Him Who really is, cannot be understood to have been born from that which was not: since He Who really is, is the cause of His existing, and His birth cannot have its origin in that which is not. And therefore since in His case it is not true either in regard of time that He ever was not, or in regard of the Father, that is, the Author of His being, that He has come into existence out of nothing, He has left no possibility with regard to Himself either of His having been born out of nothing, or of His not having existed before He was born.
Source: On the Trinity (New Advent)