Institutes of the Christian Religion (1559) - John Calvin
Book 2 - God the Redeemer
Chapter 14 - How the Two Natures of the Mediator Make One Person
Sections 4-8 - Condemnation of errors
Section 4 - The two natures may not be thought of as either fused or separated
The two natures are united: Son of God born to a virgin, the mother of our Lord - Luke 1:32, 43.
The two natures are not mixed: He called His body a temple because divinity was within - John 2:19.
Section 5 - Christ is the Son of God from everlasting
Christ "took human nature in a hypostatic union... which constitutes one person out of two natures."
He "had by virtue of eternal generation always possessed sonship."
"To neither angels nor men was God ever Father, except with regard to His only-begotten Son; and men, especially, hateful to God because of their iniquity, become God's sons by free adoption because Christ is the Son of God by nature." [Feel the zeal for Christ's glory in that sentence!]
If angels are sons of God - Ps 82:6 - and Christ is first-born of all creation - Col 1:15 - we can "infer that He was the Son of God also before the creation of the world."
Section 6 - Christ as Son of God and Son of man
Servetus says Christ couldn't have been Son of God without being human.
But He is the only-begotten because He is Son by nature - we are sons only by adoption.
Rom 1:3-4 also refutes this, implying He was named Son of God for some other reason than His taking on humanity.
Son of man refers to human nature; Son of God refers to divine nature.
Section 7 - Servetus' flimsy counterevidence
Section 8 - Comprehensive presentation and rebuttal of Servetus' doctrine