The reason for the Mediator.

John Calvin has written:

“Then when he actually appeared, he declared the cause of his advent to be, that by appeasing God he might bring us from death unto life. To the same effect was the testimony of the Apostles concerning him, (John 1: 9; 10: 14). Thus John, before teaching that the Word was made flesh, narrates the fall of man. But above all, let us listen to our Saviour himself when discoursing of his office: "God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.” Again, "The hour is coming, and now is, when the dead shall hear the voice of the Son of God: and they that hear shall live.” "I am the resurrection and the life: he that believeth in me, though he were dead, yet shall he live.” "The Son of man is come to save that which was lost.” Again, "They that be whole need not a physician.” I should never have done were I to quote all the passages. Indeed, the Apostles, with one consent, lead us back to this fountain; and assuredly, if he had not come to reconcile God, the honour of his priesthood would fall, seeing it was his office as priest to stand between God and men, and "offer both gifts and sacrifices for sins," (Heb. 5: 1); nor could he be our righteousness, as having been made a propitiation for us in order that God might not impute to us our sins, (2 Cor. 5: 19). In short, he would be stript of all the titles with which Scripture invests him. Nor could Paul's doctrine stand "What the law could not do, in that it was weak through the flesh, God sending his own Son in the likeness of sinful flesh, and for sin, condemned sin in the flesh," (Rom. 8: 3). Nor what he states in another passage: "The grace of God that bringeth salvation has appeared to all men," (Tit. 2: 11). In fine, the only end which the Scripture uniformly assigns for the Son of God voluntarily assuming our nature, and even receiving it as a command from the Father, is, that he might propitiate the Father to us by becoming a victim. "Thus it is written, and thus it behooved Christ to suffer;" - "and that repentance and remission of sins should be preached in his name.” "Therefore does my Father love me, because I lay down my life, that I might take it again.” - "This commandment have I received of my Father.” "As Moses lifted up the serpent in the wilderness, even so must the Son of man be lifted up.” "Father, save me from this hour: but for this cause came I unto this hour. Father, glorify thy name.” Here he distinctly assigns as the reason for assuming our nature, that he might become a propitiatory victim to take away sin. For the same reason Zacharias declares, (Luke 1: 79), that he came "to perform the mercy promised to our fathers," "to give light to them that sit in darkness, and in the shadow of death.” Let us remember that all these things are affirmed of the Son of God, in whom, as Paul elsewhere declares, were "hid all the treasures of wisdom and knowledge," and save whom it was his determination "not to know any thing," (Col. 2: 3; 1 Cor. 2: 2).”

(Calvin, John. Institutes of the Christian Religion, Grand Rapids, MI: Wm. B. Eerdmans Publishing, 1998, p. 403-404.)