Proof that after the Incarnation our Lord Jesus Christ, was one Son.
The authors of slanders against me allege that I divide the one Lord Jesus Christ into two sons. But so far am I from holding this opinion that I charge with impiety all who dare to say so. For I have been taught by the divine Scripture to worship one Son, our Lord Jesus Christ, the only begotten Son of God, God the Word incarnate. For we confess the same to be both God eternal, and made man in the last days for the sake of man's salvation; but made man not by the change of the Godhead but by the assumption of the manhood. For the nature of this godhead is immutable and invariable, as is that of the Father who begot Him before the ages. And whatever would be understood of the substance of the Father will also be wholly found in the substance of the only begotten; for of that substance He is begotten. This our Lord taught when He said to Philip “He that has seen me has seen the Father” and again in another place “All things that the Father has are mine,” and elsewhere “I and the Father are one,” and very many other passages may be quoted setting forth the identity of substance.
It follows that He did not become God: He was God. “In the beginning was the Word, and the Word was with God; and the Word was God.” He was not man: He became man, and He so became by taking on Him our nature: So says the blessed Paul— “Who being in the form of God thought it not robbery to be equal with God, but made Himself of no reputation, and took upon Him the form of a servant.” And again: “For verily He took not on Him the nature of angels; but He took on Him the seed of Abraham.” And again; Forasmuch then as the children are partakers of flesh and blood, He also Himself likewise took part of the same. Thus He was both passible and impassible; mortal and immortal; passible, on the one hand, and mortal, as man; impassible, on the other, and immortal, as God. As God He raised His own flesh, which was dead—as His own words declare: “Destroy this temple, and in three days I will raise it up.” And as man, He was passible and mortal up to the time of the passion. For, after the resurrection, even as man He is impassible, immortal, and incorruptible; and He discharges divine lightnings; not that according to the flesh He has been changed into the nature of Godhead, but still preserving the distinctive marks of humanity. Nor yet is His body uncircumscribed, for this is peculiar to the divine nature alone, but it abides in its former circumscription. This He teaches in the words He spoke to the disciples even after His resurrection “Behold my hands and feet that it is I myself; handle me and see; for a spirit has not flesh and bones as you see me have.” While He was thus beheld He went up into heaven; thus has He promised to come again, thus shall He be seen both by them that have believed and them that have crucified, for it is written “They shall look on Him whom they pierced.” We therefore worship the Son, but we contemplate in Him either nature in its perfection, both that which took, and that which was taken; the one of God and the other of David. For this reason also He is styled both Son of the living God and Son of David; either nature receiving its proper title. Accordingly the divine scripture calls him both God and man, and the blessed Paul exclaims “There is one God, and one mediator between God and men, the man Christ Jesus; who gave Himself a ransom for all.” But Him whom here he calls man in another place he describes as God for he says “Looking for that blessed hope and the glorious appearing of the great God and our Saviour Jesus Christ.” And yet in another place he uses both names at once saying “Of whom as concerning the flesh Christ came who is over all God blessed for ever. Amen.”
Thus he has stated the same Christ to be of the Jews according to the flesh, and God over all as God. Similarly the prophet Isaiah writes “A man of sorrows and acquainted with grief....Surely He has borne our griefs and carried our sorrows,” and shortly afterwards he says “Who shall declare His generation?” This is spoken not of man but of God. Thus through Micah God says “Thou Bethlehem in the land of Judah art not the least among the princes of Judah, for out of you shall come a governor that shall rule my people Israel, whose goings forth have been as of old from everlasting.” Now by saying “From you shall come forth a ruler” he exhibits the œconomy of the incarnation; and by adding “whose goings forth have been as of old from everlasting” he declares the Godhead begotten of the Father before the ages.
Since we have been thus taught by the divine scripture, and have further found that the teachers who have been at different periods illustrious in the Church, are of the same opinion, we do our best to keep our heritage inviolate; worshipping one Son of God, one God the Father, and one Holy Ghost; but at the same time recognising the distinction between flesh and Godhead. And as we assert them that divide our one Lord Jesus Christ into two sons to trangress from the road trodden by the holy apostles, so do we declare the maintainers of the doctrine that the Godhead of the only begotten and the manhood have been made one nature to fall headlong into the opposite ravine. These doctrines we hold; these we preach; for these we do battle.
The slander of the libellers that represent me as worshipping two sons is refuted by the plain facts of the case. I teach all persons who come to holy Baptism the faith put forth at Nicæa; and, when I celebrate the sacrament of regeneration I baptize them that make profession of their faith in the name of the Father, and of the Son, and of the Holy Ghost, pronouncing each name by itself. And when I am performing divine service in the churches it is my wont to give glory to the Father and to the Son and to the Holy Ghost; not sons, but Son. If then I uphold two sons, whether of the two is glorified by me, and whether remains unhonoured? For I have not quite come to such a pitch of stupidity as to acknowledge two sons and leave one of them without any tribute of respect. It follows then even from this fact that the slander is proved slander—for I worship one only begotten Son, God the Word incarnate. And I call the holy Virgin “Mother of God” because she has given birth to the Emmanuel, which means “God with us.” But the prophet who predicted the Emmanuel a little further on has written of him that “Unto us a child is born, unto us a son is given; and the government shall be upon his shoulders; and his name is called Angel of great counsel, wonderful, counsellor, mighty God, powerful, Prince of peace, Father of the age to come.” Now if the babe born of the Virgin is styled “Mighty God,” then it is only with reason that the mother is called “Mother of God.” For the mother shares the honour of her offspring, and the Virgin is both mother of the Lord Christ as man, and again is His servant as Lord and Creator and God.
Source: Letters (New Advent)