Why did Jesus have to become a man? Couldn’t He, in His omnipotence, have arranged our redemption without leaving heaven? In Christ’s priestly work we find the answer to that question.
A priest has to partake of the nature of the persons for whom he officiates. A true high priest, therefore, had to be “taken from among men”—that is, he had to be a man. God did not choose angels to be priests. Angels do not have the nature of men. They cannot truly understand men, and they do not have open communication with men. Only a man could be subject to the temptations of men, could experience suffering like men, and thereby be able to minister to men in an understanding and merciful way. Only a man could rightly minister on behalf of men.
For every high priest taken from among men is appointed on behalf of men in things pertaining to God, in order to offer both gifts and sacrifices for sins; he can deal gently with the ignorant and misguided, since he himself also is beset with weakness; and because of it he is obligated to offer sacrifices for sins, as for the people, so also for himself. And no one takes the honor to himself, but receives it when he is called by God, even as Aaron was. (Hebrews 5:1–4)
Remembering to whom the book of Hebrews was written, we can more easily see the importance of the point being made here about Jesus Christ. To be the perfect High Priest, in fact to be a high priest at all, He had to be a man. This much, of course, was completely clear and acceptable to Jews. Their problem was with the incarnation—God’s becoming a man. The Holy Spirit very simply answers the problem of the incarnation with the one basic point: The Messiah, who is God, could not be a true high priest unless He were a man. Unless God could feel what men feel and go through what men go through, He would have no experiential understanding of those whom He represented.
Under the old economy, even after the covenants with Abraham and with Moses, God was unapproachable. At the Fall, God had driven Adam and Eve out of the garden, and man no longer had access to the Lord’s presence. In the wilderness, the people were warned not to come too near Sinai, where God chose to manifest Himself to Moses when giving the covenant of the law. In the Tabernacle and in the Temple, God was behind a veil and could be approached only throuqh a high priest.
But in sending His Son, Jesus Christ, God no longer kept Himself aloof, transcendent, and separate from men. He entered into the human world and felt everything that men will ever feel—thus He is our sympathetic, merciful, and faithful High Priest. If God had never become man, He never could have been a high priest, a mediator, or an intercessor. He never could have offered the perfect and absolute sacrifice for the sins of His people, that divine justice required. The incarnation was not an option; it was an absolute necessity. It was an imperative if men were to be saved.
John Calvin said,
It follows that Christ must have been truly man. Because we stand a long way off from God, we are in some way placed before Him in His priestly character. This could not be so if He were not one of us. The fact that the Son of God has a common nature with us does not detract from His dignity, but rather commends Him the more to us. He is fitted to reconcile God to us because he is man.  John Calvin, Calvin’s Commentaries: The Epistle of Paul the Apostle to the Hebrews and the First and Second Epistles of St. Peter, trans. by William B. Johnston (Grand Rapids, MI: Wm. B. Eerdmans Publishing Company, 1963), 58–59.
God had to come down to where we are in order to pick us up and bring us back to Himself.
But a true priest could not be just any man. He had to be appointed by God. It was not an office that a man could fill simply because of his own plans or ambition. He had to be God’s man—not simply in the sense of being faithful and obedient to God, but in the sense of being selected by God. He was appointed on behalf of men, but by God. “No one takes the honor to himself, but receives it when he is called by God, even as Aaron was” (Hebrews 5:4; cf. 8:3).
When the priesthood was first established, Moses was instructed, “Bring near to yourself Aaron your brother, and his sons with him, from among the sons of Israel, to minister as priest to Me—Aaron, Nadab and Abihu, Eleazar and Ithamar, Aaron’s sons” (Exodus 28:1). From the beginning of the priesthood, the priests not only were to minister for God but by His appointment. When Korah, Dathan, Abiram, and On insisted on trying to democratize the priesthood and claimed that any Israelite could be a priest, the Lord caused the earth to swallow them up (Numbers 16).
In Christ, we have the perfect High Priest to represent us before God the Father. He totally fulfills every divine requirement and completely understands every human frailty. He completely knows and understands what we go through as His people living in this fallen world. He lived it.
(Adapted from The MacArthur New Testament Commentary: Hebrews)