34 In this treatise we have followed the course which we know is pursued in every branch of education. First come easy lessons and a familiarity, slowly attained by practice, with the groundwork of the subject; then the student may make proof, in the business of life, of the training which he has received. Thus the soldier, when he is perfect in his exercises, can go out to battle; the advocate ventures into the conflicts of the courts when he is versed in the pleadings of the school of rhetoric; the sailor who has learned to navigate his ship in the land-locked harbour of his home may be trusted amid the storms of open seas and distant climes.
Such has been our proceeding in this most serious and difficult science in which the whole faith is taught. First came simple instruction for the untaught believer in the birth, the name, the Divinity, the true Divinity of Christ; since then we have quietly and steadily advanced till our readers can demolish every plea of the heretics; and now at last we have pitted them against the adversary in the present great and glorious conflict. The mind of men is powerless with the ordinary resources of unaided reason to grasp the idea of an eternal birth, but they attain by study of things Divine to the apprehension of mysteries which lie beyond the range of common thought.
They can explode that paradox concerning the Lord Jesus, which derives all its strength and semblance of cogency from a purblind pagan philosophy: the paradox which asserts, There was a time when He was not, and He was not before He was born, and He was made out of nothing; as though His birth were proof that He had previously been non-existent and at a given moment came into being, and God the Only-begotten could thus be subjected to the conception of time, as if the faith itself [by conferring the title of 'Son'] and the very nature of birth proved that there was a time when He was not.
Accordingly they argue that He was born out of nothing, on the ground that birth implies the grant of being to that which previously had no being. We proclaim in answer, on the evidence of Apostles and Evangelists, that the Father is eternal and the Son eternal, and demonstrate that the Son is God of all with an absolute, not a limited, pre-existence; that these bold assaults of their blasphemous logic— He was born out of nothing, and He was not before He was born— are powerless against Him; that His eternity is consistent with sonship, and His sonship with eternity; that there was in Him no unique exemption from birth but a birth from everlasting, for, while birth implies a Father, Divinity is inseparable from eternity.
Source: On the Trinity (New Advent)