Nicene and Post-Nicene Fathers, Ser. II, Vol. XI:
The Works of John Cassian.: Chapter I. That Christ was before the Incarnation God from everlasting.

Chapter I.

That Christ was before the Incarnation God from everlasting.

As we have finished three books with the most certain and the most valuable witnesses, whose truth is substantiated not only by human but also by Divine evidences, they would abundantly suffice to prove our case by Divine authority, especially as the Divine authority of the case itself would be enough for this. But still as the whole mass of the sacred Scriptures is full of these evidences, and where there are so many witnesses, there are so many opinions to be urged—nay where Holy Scripture itself gives its witness so to speak with one Divine mouth—we have thought it well to add some others still, not from any need of confirmation, but because of the supply of material at our disposal; so that anything which might be unnecessary for purposes of defence, might be useful by way of ornamentation. Therefore since in the earlier books we proved the Divinity of our Lord Jesus Christ while He was in the flesh by the evidence not only of prophets and apostles, but of evangelists and angels as well, let us now show that He who was born in the flesh was God even before His Incarnation; that you may understand by the harmony and concord of the evidences from the sacred Scriptures, that you ought to believe that at His birth in the body He was both God and man, who before His birth was only God, and that He who after He had been brought forth by the Virgin in the body was God, was before His birth from the Virgin, God the Word. Learn then first of all from the Apostle the teacher of the whole world, that He who is without beginning, God, the Son of God, became the Son of man at the end of the world, i.e., in the fulness of the times. For he says: “But when the fulness of the times was come, God sent His Son, made of a woman, made p. 574 under the law.” 2448 Tell me then, before the Lord Jesus Christ was born of His mother Mary, had God a Son or had He not? You cannot deny that He had, for never yet was there either a son without a father, or a father without a son: because as a son is so called with reference to a father, so is a father so named with reference to a son.

Footnotes

Gal. iv. 4.