Theodore Beza (1519-1605) here speaks of the necessity of Christ being both God and man, in many ways resonating the Heidelberg Catechism Lord’s Day 6. These quotes are from Beza’s The Christian Faith (Confession De Foi Du Chretien), p. 11-12:
Why it was necessary that Jesus Christ be true man in nature, in His body and in His soul, but without any sin:
“It was necessary that the Mediator of this covenant and this reconciliation be true man, but without any stain of original sin or any other, for the following reasons:
Firstly, since God is very righteous and man is the object of His wrath, because of natural corruption (1 Tim 2:5; John 1:14; Rom 1:3; Gal 4:4; Rom 8:2-4; 1 Cor. 1:30), it was necessary in order to reconcile men with God, that there be a true man in whom the ruins caused by this corruption would be totally repaired.
Secondly, man is compelled to fulfill all the righteousness which God demands from him in order to be glorified (Matt 3:15; Rom 5:18; 2 Cor. 5:21). It was therefore necessary that there be a man who would perfectly fulfill all righteousness in order to please God.
Thirdly, all men are covered with an infinite number of sins, as much internal as external; that is why they are liable to the curse of God (Rom 3:23-26; Is 53: 11, etc). It was therefore necessary that there be a man who would fully satisfy the justice of God in order to pacify Him.
Finally, no corrupt man would have been able, in any way, to even begin to fulfill the least of these actions. He would first of all have had need of a Redeemer for himself (Rom 8:2; 2 Cor. 5:21; Heb 4:15; 1 Pet 2:22; 3:18; 1 John 2:1-2). So much was necessary for himself before he could buy back the others, or could do anything pleasing or satisfying to God (Rom 14:23; Heb 11:6). It was therefore necessary that the Mediator and Redeemer of men be true man in his body and in his soul, and that he be, nevertheless, entirely pure and free from all sin.”
Why it was necessary that Jesus Christ be true God:
“It was necessary that this same Mediator be true God and not only man (John 1:14, etc); at the very least for the following reasons:
Firstly, if He was not true God, He would not be Saviour at all, but would himself have need of a Saviour (Is 43:11; Hos. 13:4; Jer. 17:5-8).
Secondly, it is necessary, from the justice of God, that there be a relationship between the crime and its punishment. The crime is infinite, for it is committed against One whose majesty is infinite. Therefore there is here need of an infinite satisfaction; for the same reason, it was necessary that the One who would accomplish it as true man be also infinite, that is to say, true God.
Thirdly, the wrath of God being infinite, there was no human or angelic strength known which could bear such a weight without being crushed (John 14:10,12,31; 16:32; 2 Cor. 5:19). He who was to live again, after having conquered the devil, sin, the world and death united to the wrath of God, had to be therefore not only perfect man, but also true God.
Lastly, in order to better manifest this incomprehensible goodness, God did not wish that His grace should only equal our crime; He willed that where sin abounds, grace superabounds (Rom 5:15-21). For this reason, while he was created in the image of God, the first Adam, author of our sin, was earthly, as his frailty showed well (1 Cor. 15:45-47). Jesus Christ, on the contrary, the second Adam, through whom we are saved, while being true and perfect man, is nevertheless the Lord come from Heaven, that is to say, the true God. For, in essence, all the fullness of divinity dwells in Him (Col. 2:9). If the disobedience of Adam made us fall, the righteousness of Jesus Christ gives us more security than we had previously. We hope for life procured by Jesus Christ, better than that which we lost in Adam; even more so as Jesus Christ surpasses Adam.”