Proof that after the Incarnation our Lord Jesus Christ, was one Son.
On account of this difference of term He is said by the divine Paul to be “without father, without mother, without descent, having neither beginning of days nor end of life.” He is without father as touching His humanity; for as man He was born of a mother alone. And He is without mother as God, for He was begotten from everlasting of the Father alone. And again He is without descent as God while as man He has descent. For it is written “The book of the generation of Jesus Christ the son of David, the son of Abraham.” His descent is also given by the divine Luke. So again, as God, He has no beginning of days for He was begotten before the ages; neither has He an end of life, for His nature is immortal and impassible. But as man He had both a beginning of days, for He was born in the reign of Augustus Cæsar, and an end of life, for He was crucified in the reign of Tiberius Cæsar. But now, as I have already said, even His human nature is immortal; and, as He ascended, so again shall He come according to the words of the Angel— “This same Jesus which is taken up from you into Heaven shall so come in like manner as you have seen Him go into Heaven.”
This is the doctrine delivered to us by the divine prophets; this is the doctrine of the company of the holy apostles; this is the doctrine of the great saints of the East and of the West; of the far-famed Ignatius, who received his archpriesthood by the right hand of the great Peter, and for the sake of his confession of Christ was devoured by savage beasts; and of the great Eustathius, who presided over the assembled council, and on account of his fiery zeal for true religion was driven into exile. This doctrine was preached by the illustrious Meletius, at the cost of no less pains, for thrice was he driven from his flock in the cause of the apostles' doctrines; by Flavianus, glory of the imperial see; and by the admirable Ephraim, instrument of divine grace, who has left us in the Syriac tongue a written heritage of good things; by Cyprian, the illustrious ruler of Carthage and of all Libya, who for Christ's sake found a death in the fire; by Damasus, bishop of great Rome, and by Ambrose, glory of Milan, who preached and wrote it in the language of Rome.
The same was taught by the great luminaries of Alexandria, Alexander and Athanasius, men of one mind, who underwent sufferings celebrated throughout the world. This was the pasture given to their flocks by the great teachers of the imperial city, by Gregory, shining friend and supporter of the truth; by John, teacher of the world, by Atticus, their successor alike in see and in sentiment. By these doctrines Basil, great light of the truth, and Gregory sprung from the same parents, and Amphilochius, who from him received the gift of the high-priesthood, taught their contemporaries, and have left the same to us in their writings for a goodly heritage. Time would fail me to tell of Polycarp, and Irenæus, of Methodius and Hippolytus, and the rest of the teachers of the Church. In a word I assert that I follow the divine oracles and at the same time all these saints. By the grace of the spirit they dived into the depths of God-inspired scripture and both themselves perceived its mind, and made it plain to all that are willing to learn. Difference in tongue has wrought no difference in doctrine, for they were channels of the grace of the divine spirit, using the stream from one and the same fount.
Source: Letters (New Advent)