34 But, heretic, do you consider it pious and devout to confess that God indeed always existed, yet was not always Father? For if it is pious for you to think so, you must then condemn Paul of impiety, when he says that the Son existed before times eternal: you must also accuse Wisdom itself, when it bears witness concerning itself that it was founded before the ages: for it was present with the Father when He was preparing the heaven. But in order that you may assign to God a beginning of His being a Father, first determine the starting-point at which the times must have begun.
For if they had a beginning, the Apostle is a liar for declaring them to be eternal. For you all are accustomed to reckon the times from the creation of the sun and the moon, since it is written of them, And let them be for signs and for times and for years. But He Who is before the heaven, which in your view is even before time, is also before the ages. Nor is He merely before the ages, but also before the generations of generations which precede the ages. Why do you limit things divine and infinite by what is perishable and earthly and narrow?
With regard to Christ, Paul knows of nothing except an eternity of times. Wisdom does not say that it is after anything, but before everything. In your judgment the times were established by the sun and the moon; but David shows that Christ remains before the sun, saying, His is name is before the sun. And lest you should think that the things of God began with the formation of this universe, he says again, And for generations of generations before the moon. These great men counted worthy of prophetic inspiration look down upon time: every opening is barred whereby human perception might penetrate behind the birth, which transcends times eternal.
Yet let the faith of a devout imagination accept this as limit of its speculations, remembering that the Lord Jesus Christ, God Only-begotten, is born in a manner to be acknowledged as a perfect birth, and in the reverence paid to His divinity, not forgetting that He is eternal.
Source: On the Trinity (New Advent)