51 But none of these phrases does a firm apostolic faith permit. For it knows in what dispensation of time Christ was created, and in what eternity of times He was born. Moreover, He was born God of God, and the divinity of His true birth and perfect generation is not doubtful. For in relation to God we acknowledge only two modes of being, birth and eternity: birth, moreover, not after anything, but before all things, so that birth only bears witness to a Source of being, and does not predicate any incongruity between the offspring and the Source of being.
Still, by common admission, this birth, because it is from God, implies a secondary position in respect to the Source of being, and yet cannot be separated from that Source, since any attempt of thought to pass beyond acceptance of the fact of birth, must also necessarily penetrate the mystery of the generation. And so this is the only pious language to use about God: to know Him as Father, and with Him to know also Him, Who is the Son born of Him. Nor assuredly are we taught anything concerning God, except that He is the Father of God the Only-begotten and the Creator.
So let not human weakness overreach itself; and let it make this only confession, in which alone lies its salvation— that, before the mystery of the Incarnation, it is ever assured, concerning the Lord Jesus Christ, of this one fact that He had been born.
Source: On the Trinity (New Advent)