For every high priest taken from men is appointed in service to God for the people, to offer both gifts and sacrifices for sins. He is able to deal gently with those who are ignorant and are going astray, since he is also subject to weakness. Because of this, he must make a sin offering for himself as well as for the people. No one takes this honor on himself; instead, a person is called by God, just as Aaron was. In the same way, the Messiah did not exalt Himself to become a high priest, but the One who said to Him, You are My Son; today I have become Your Father, also said in another passage, You are a priest forever in the order of Melchizedek.
During His earthly life, He offered prayers and appeals with loud cries and tears to the One who was able to save Him from death, and He was heard because of His reverence. Though He was God’s Son, He learned obedience through what He suffered. After He was perfected, He became the source of eternal salvation for all who obey Him, and He was declared by God a high priest in the order of Melchizedek. – Hebrews 5:1-10 (HCSB)
Having demonstrated the reality of Christ’s dual natures, and the implications for soteriology, our author moves to begin to explain the mechanics of salvation in light of those realities.
A fundamental element of typology is that the antitype is always greater than the type. The author here begins to demonstrate that the priesthood of Christ is better than the levitical priesthood. He also shows ways that his priesthood is both the same, and different.
First he notes that the high priest is appointed from among the people on whose behalf he will be mediating. This is in order that this high priest can properly represent and sympathize with them. However, because he is chosen from among weak and sinful humanity he must not only offer atonement for the people but also for his own sin.
This high priest does not choose the role himself, but only God can ordain a man to this service.
Staring first with the way that Christ’s priesthood is the same he notes that the Son did not choose this priesthood. Rather, the Father appointed it for him. While we do not precisely understand how this could be in eternity past, the text never-the-less indicates that the Son was appointed by the Father to be the high priest and redeemer of the redeemed.
While the ongoing and eternal mediatorial work of the Son is a theme in the book of Hebrews, we see here that the Son’s priesthood began during his earthly ministry. However, rather than offer lambs and goats, he offered prayers and appeals to the Father on behalf of his people. Far too often we think of the Son’s sacrifice exclusively in terms of his death on the cross, but Christ’s passive obedience and suffering began upon the incarnation.
As we noted before, the Son has two distinct relationships with the Father. One is his eternal, immutable, and natural sonship. That is, the relationship that the Son had with the Father in light of their shared nature. This filial union never had a beginning, was never broken, and will never change. In addition, he possesses an adopted sonship which he earned by means of his active obedience. This is what our author speaks of when he says “learned obedience” and “after he was perfected.” (contra adoptionism/inspirational Christology) Having obtained this adopted relationship by means of suffering and obedience he can then grant to us that very relationship when we embrace him by faith. These dual relationships are echoed by what we see here in the text as a sort of dual priesthood. While we see that in eternity past the Son was already a priest forever in the order of Melchizedek, upon being perfected according to his humanity he was also declared to be a high priest in the order of Melchizedek.
 This is a repeated pattern in the New Testament, that what the Son always was qua divinity in relationship to the Father, he obtains qua humanity through obedience and then grants to the Elect. This pattern is at the core of the Patristic doctrine of theosis or divinization and is especially clear in Athanasius of Alexandria and Cyril of Alexandria. While the Eastern Orthodox expression of this ancient doctrine took a decidedly ontological turn following Maximus and especially Palamas, it was recovered by Reformed thinkers and finds its fullest and most comprehensive expression in Covenant Theology. The CT expression of this concept takes the following form:
Adam was promised by gracious adoption, the filial relationship the Son possessed with the Father by nature. Had he fulfilled the terms of the Covenant of Works/Creation, all of those for whom Adam was the federal head would have shared in that blessing. Because he failed, the Second Adam took on flesh and obtained that adopted relationship which he now grants to all of those for whom he is the federal head. We transfer our federal headship from Adam to the Second Adam by embracing the Second Adam by faith.