John Calvin on the Incarnation and the Two Natures of Christ

They must certainly admit, that as on account of his mother he is called the Son of David, so, on account of his Father, he is the Son of God, and that in some respect differing from his human nature. The Scripture gives him both names, calling him at one time the Son of God, at another the Son of Man.  --John Calvin

6. But if his filiation (if I may so express it) had a beginning at the time when he was manifested in the flesh, it follows that he was a Son in respect of human nature also. Servetus, and others similarly frenzied, hold that Christ who appeared in the flesh is the Son of God, inasmuch as but for his incarnation he could not have possessed this name. Let them now answer me, whether, according to both natures, and in respect of both, he is a Son? So indeed they prate; but Paul’s doctrine is very different. We acknowledge, indeed, that Christ in human nature is called a Son, not like believers by gratuitous adoption merely, but the true, natural, and, therefore, only Son, this being the mark which distinguishes him from all others. Those of us who are regenerated to a new life God honours with the name of sons; the name of true and only-begotten Son he bestows on Christ alone. But how is he an only Son in so great a multitude of brethren, except that he possesses by nature what we acquire by gift? This honour we extend to his whole character of Mediator, so that He who was born of a Virgin, and on the cross offered himself in sacrifice to the Father, is truly and properly the Son of God; but still in respect of his Godhead: as Paul teaches when he says, that he was “separated unto the gospel of God (which he had promised afore by his prophets in the Holy Scriptures), concerning his Son Jesus Christ our Lord, which was made of the seed of David according to the flesh; and declared to be the Son of God with power,” ( [Rom. 1:1–4] ). When distinctly calling him the Son of David according to the flesh, why should he also say that he was “declared to be the Son of God,” if he meant not to intimate, that this depended on something else than his incarnation? For in the same sense in which he elsewhere says, that “though he was crucified through weakness, yet he liveth by the power of God,” ( [2 Cor. 13:4] ), so he now draws a distinction between the two natures. They must certainly admit, that as on account of his mother he is called the Son of David, so, on account of his Father, he is the Son of God, and that in some respect differing from his human nature. The Scripture gives him both names, calling him at one time the Son of God, at another the Son of Man. As to the latter, there can be no question that he is called a Son in accordance with the phraseology of the Hebrew language, because he is of the offspring of Adam. On the other hand, I maintain that he is called a Son on account of his Godhead and eternal essence, because it is no less congruous to refer to his divine nature his being called the Son of God, than to refer to his human nature his being called the Son of Man. In fine, in the passage which I have quoted, Paul does not mean, that he who according to the flesh was begotten of the seed of David, was declared to be the Son of God in any other sense than he elsewhere teaches that Christ, who descended of the Jews according to the flesh, is “over all, God blessed for ever,” ( [Rom. 9:5] ). But if in both passages the distinction of two natures is pointed out, how can it be denied, that he who according to the flesh is the Son of Man, is also in respect of his divine nature the Son of God?

Institutes of the Christian Religion, Book 2: Ch. 14: Section 6