Herman Witsius: Our Mediator Must Be Both God And Man

Had he been God only he could neither have been subject, nor have obeyed, nor have suffered: if mere man, his obedience, subjection, and suffering, would not have been of sufficient value for the redemption of the elect. Nay, a mere creature is so bound to fulfill all righteousness for itself, that its righteousness cannot be imputed and imparted to others: and should we suppose a man truly and perfectly holy, but yet a mere man, who, according to the law of love, offered himself even to die for his brother, he himself would doubtless obtain a reward by his righteousness; but could merit nothing for a guilty person, unless perhaps exemption from punishment at most. And therefore it behooved our Surety to be man, that he might be capable to submit, obey, and suffer; and at the same time God, that the subjection, obedience, and suffering, of this person God-man, might on account of his infinite dignity, be imputed to others, and be sufficient for saving all, to who it is imputed. 

Economy of the Covenants Between God and Man 2.4.20