John Calvin’s Institutes, Book II, chapter 12, section 1:
It deeply concerned us, that he who was to be our Mediator should be very God and very man. If the necessity be inquired into, it was not what is commonly termed simple or absolute, but flowed from the divine decree on which the salvation of man depended. What was best for us, our most merciful Father determined? Our iniquities, like a cloud intervening between Him and us, having utterly alienated us from the kingdom of heaven, none but a person reaching to him could be the medium of restoring peace. But who could thus reach to him? Could any of the sons of Adam? All of them, with their parents, shuddered at the sight of God. Could any of the angels? They had need of a head, by connection with which they might adhere to their God entirely and inseparably.
What then? The case was certainly desperate, if the Godhead itself did not descend to us, it being impossible for us to ascend. Thus the Son of God behooved to become our Emmanuel, the God with us; and in such a way, that by mutual union his divinity and our nature might be combined; otherwise, neither was the proximity near enough, nor the affinity strong enough, to give us hope that God would dwell with us; so great was the repugnance between our pollution and the spotless purity of God. Had man remained free from all taint, he was of too humble a condition to penetrate to God without a Mediator. What, then, must it have been, when by fatal ruin he was plunged into death and hell, defiled by so many stains, made loathsome by corruption; in fine, overwhelmed with every curse? It is not without cause, therefore, that Paul, when he would set forth Christ as the Mediator, distinctly declares him to be man. There is, says he, “one Mediator between God and man, the man Christ Jesus,” (1 Tim. 2: 5). He might have called him God, or at least, omitting to call him God he might also have omitted to call him man; but because the Spirit, speaking by his mouth, knew our infirmity, he opportunely provides for it by the most appropriate remedy, setting the Son of God familiarly before us as one of ourselves. That no one, therefore, may feel perplexed where to seek the Mediator, or by what means to reach him, the Spirit, by calling him man, reminds us that he is near, nay, contiguous to us, inasmuch as he is our flesh. And, indeed, he intimates the same thing in another place, where he explains at greater length that he is not a high priest who “cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin,” (Heb. 4: 15)