26 And so we confess that God Only-begotten was born, but born before times eternal: since we must make our confession within such limits as the express preaching of Apostles and Prophets assigns to us; though at the same time human thought cannot grasp any intelligible idea of birth out of time, since it is inconsistent with the nature of earthly beings that any of them should be born before all times. But when we make this assertion, how can we reconcile with it, as part of the same doctrine, the contradictory statement that before His birth He was not, when according to the Apostle He is God Only-begotten before times eternal?
If, therefore, the belief that He was born before times eternal is not only the reasonable conclusion of human intelligence, but the confession of thoughtful faith, then, since birth implies some author of being, and what surpasses all time is eternal, and whatever is born before times eternal transcends earthly perception, we are certainly exalting by impious self-will a notion of human reason, if we maintain in a carnal sense that before He was born He was not, since He is born eternal, beyond human perception or carnal intelligence. And again, whatever transcends time is eternal.
Source: On the Trinity (New Advent)