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 fulfil 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 fulfil 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 fulfil 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.
— Theodore Beza (1519-1605)