NOW IF PERFECTION WAS THROUGH THE LEVITICAL PRIESTHOOD: ei men oun teleiosis dia tes leuitikes hierosunes en (3SIAI): (Heb 7:18,19; 8:7,10-13; 10:1-4; Galatians 2:21; 4:3,9; Colossians 2:10-17)

Ray Stedman explains that...

The argument of Hebrews 7:11-19 constitutes a bold, and even radical, declaration by the writer. This section asserts unequivocally that the death and resurrection of Jesus has introduced a new and permanent priesthood that brings the Levitical priesthood to an end and, with it, the demise of the law of Moses. It is important to note in Hebrews 7:11-12 that the law was originally given to support the priesthood, not the other way around. The priesthood and the tabernacle with its sacrifices were the means God employed to render the sinful people acceptable to himself. They constituted the shadow of Jesus in the Old Testament. Then the law was given with its sharp demands to awaken the people to their true condition so that they might avail themselves of the sacrifices. This agrees fully with Paul’s state ment in Romans 5:20 and Galatians 3:19-23 that the law was a teacher to lead to Christ (represented in Israel by the tabernacle and its priesthood).

To suggest that either of these venerable institutions (the priesthood and the law) were inadequate and needed change was to assault Judaism in its most sacred and revered precincts. But that this was the teaching of Christians from the beginning is seen in the savage charges hurled at Stephen, and later Paul, when they engaged certain Jewish leaders in religious dialog. See, for instance, Acts 6:14, where Stephen’s opponents testified, “We have heard him say that this Jesus of Nazareth will destroy this place [the temple] and change the customs Moses handed down to us.” (Hebrews 7:11-19 Aaronic Priesthood and Law Replaced)

Perfection - in context means to put someone in the position in which he can come before God.

An institution is perfect or complete when it effects the purpose for which it was instituted, and produces a result that corresponds to the idea of it. The purpose of the priesthood was to remove the obstacle, sin, which kept man from God, and make a way of access for man to God. The Levitical priesthood could do that in a typical but not in an actual (perfect) way. The priesthood and the sacrifices were an index finger pointing to the Messiah and His substitutionary death on the Cross.

The purpose of the priesthood was to reconcile men to God through sacrificing for their sins. But this priesthood could only picture the actual reconciliation, because it could only typify cleansing of sin. The OT system was not able to take away sin. It was therefore imperfect. The Levitical priesthood failed to give men a perfectly adequate relation to God and failed to give men access to God, which is the goal for men who are created in God's image.

FOR ON THE BASIS OF IT THE PEOPLE RECEIVED THE LAW: o laos gar ep aute nenomotheteto

John MacArthur explains that...

What the old economy could not do, Christ did. The old priesthood had its place in God’s plan. But it was inferior and ineffective. It only pictured perfection. Similarly, the law had its place in God’s plan. It represented God’s truth and righteousness. It demanded perfection. For on the basis of it [that is, perfection] the people received the Law. But neither the sacrifice that pictured it nor the law that demanded it could provide perfection. Perfection is provided only in Jesus Christ.

Received the law (3549) (nomotheteo from nómos = law + títhemi = to put, set) literally means the law has been laid down and thus means established as law. 

The Law had profound limitations. It could not atone for sin. The Levitical sacrifices covered over sin, but they did not remove it. The Law marvelously served to enhance one’s awareness of sin. Paul tells us in Ro 7:7, 8 that the Law’s command not to covet made him aware that all he did was covet. The Law was a teacher Gal3:24 The law was powerless… in that it was weakened by the sinful nature… the sinful mind is hostile to God. It does not submit to God’s law, nor can it do so” (Ro 8:3, 7).

F. F. Bruce says,

The whole apparatus of worship associated with sacrifice and ritual and priesthood was calculated rather to keep men at a distance from God than to bring them near.

Clearly, the Old Covenant had profound limitations as to making atonement, imparting life, clearing the conscience, and providing access.

WHAT FURTHER NEED WAS THERE FOR ANOTHER PRIEST TO ARISE ACCORDING TO THE ORDER OF MELCHIZEDEK AND NOT BE DESIGNATED ACCORDING TO THE ORDER OF AARON: tis eti chreia heteron anistasthai hierea kata ten taxin melchisedek heteron anistasthai hierea kai ou kata ten taxin aaron legesthai (PPN): (Heb 7:15,17,21; 5:6,10; 6:20)

Further (eti) speaks of an extension of time up to and beyond an expected point. If there was perfection through the Levitical priesthood and if the old sacrifices had been able to bring a person into God’s presence, they would have ceased. They would have fulfilled their purpose.

The OT saints lacked the total sense of freedom from the consciousness of their sin. They came short of that full privilege, because the sacrifices of that covenant could not completely remove their sin and bring them to God. Because their sins were not finally cleansed, their consciences could not be wholly cleansed, could not be freed. The New Covenant gives greater understanding of full forgiveness, freedom from guilt, and a peaceful conscience.

FOR WHEN THE PRIESTHOOD IS CHANGED OF NECESSITY THERE TAKES PLACE A CHANGE OF LAW ALSO: metatithemenes (PPPFSG) gar tes hierosunes ex anagkes kai nomou metathesis ginetai (3SPMI): (Isaiah 66:21; Jeremiah 31:31-34; Ezekiel 16:61; Acts 6:13,14)

When the priesthood is changed - transferred from the Levitical (Law) system of priesthood to that of the order of Melchizedek in Christ. God's choice of this new type of priesthood for His Son, left the Levitical line off to one side, forever discounted, passed by "the order of Aaron".

Christianity, in a sense, comes from Judaism. But Christianity is not merely enhanced Judaism; it replaces Judaism. For a Jewish convert, his faith is changed from Judaism to Christianity. The new priesthood, after the order of Melchizedek, was not added to Aaron’s, but replaced it.

Change (3331) (metathesis from metatíthemi = transfer from meta  = implying change + tithemi = put) is literally, the act of transferring from one place to another  and so the removal or taking up or away. And so it can describe a transfer from one place to another, as for example the translation of a person to heaven...

By faith Enoch was taken up so that he should not see death; and he was not found because God took him up; for he obtained the witness that before his being taken up he was pleasing to God. (see note Hebrews 11:5).

Hebrews 12:27 uses metathesis with the idea of removal.

And this expression, "Yet once more," denotes the removing of those things which can be shaken, as of created things, in order that those things which cannot be shaken may remain.

Figuratively, metathesis means to transpose or put one thing in the place of another. It can mean a change of things instituted or established, such as a changeover from the Levitical priesthood

For when the priesthood is changed, of necessity there takes place a change of law also. (Hebrews 7:12)

Transliterated it gives us the English word metathesis which is defined as the transposition of a letter of a word.

Vincent comments on the meaning of metathesis here in Hebrews 7:12 writing that it refers to

A change. A transfer to a new basis. Only in Hebrews. See notes Hebrews 11:5; Hebrews 12:27. The inferiority of the Levitical priesthood is inferred from the fact that another priesthood was promised. If perfection was possible at all under the Mosaic economy, it must come through the Levitical priesthood, since that priesthood was, in a sense, the basis of the law. The whole legal system centered in it. The fundamental idea of the law was that of a people united with God. Sin, the obstacle to this ideal union, was dealt with through the priesthood. If the law failed to effect complete fellowship with God, the priesthood was shown to be a failure, and must be abolished; and the change of the priesthood involved the abolition of the entire legal system.

Wuest explains the transfer to a new basis noting that

The priesthood after the order of Melchizedek was put in the place of the priesthood after the order of Aaron. The blood of animals could not pay for sin, but the blood of Messiah could. Thus, the New Covenant was substituted for the Old Covenant, Jesus’ blood, the reality, for animal blood, the type. But that could only be done by changing the law governing the priesthood. Thus, if a transfer to a new and different order of priesthood was to be effected, it must be by reason of a transfer to a new basis. The law governing the priesthood as found in the Mosaic economy must be abrogated in favor of another which would provide for an order of priesthood that would function successfully in the very thing in which the Aaronic priesthood failed.- Wuest)

The inferiority of the Levitical priesthood is inferred from the fact that another priesthood was promised. If perfection was possible at all under the Mosaic economy, it must come thru the Levitical priesthood, since that priesthood was, in a sense, the basis of the law. The whole legal system centered in it. The fundamental idea of the law was that of a people united w God. Sin, the obstacle to this ideal union, was dealt with through the priesthood. If the law failed to effect complete fellowship w God, the priesthood was shown to be a failure, and must be abolished; and the change of the priesthood involved the abolition of the legal (ceremonial) system, not so much the 10 Commandments (in fact those standards remained unchanged in the New Covenant where the Law was now written on man's heart) but the ceremonial law, the Aaronic system of sacrifices, the required rituals and ceremonies of the Old Covenant, has been set aside. Setting this aside was extremely difficult for many Jews to do and the reasons for doing so were extremely hard to grasp.

Some believing Jews, in fact, not only insisted on maintaining their own Jewish practices, but on making them mandatory for everyone who wanted to become a Christian. These people were called Judaizers, and they were a plague to the early church for many years. They told prospective believers, and even non-Jewish Christians, that they needed to be circumcised and have sacrifices made in the Temple and follow all the prescribed Jewish laws and rituals. Paul directly attacked these false teachers in Gal 3:1, 3, 5; 4:9; 5:6.

At Sinai the people were fenced off at the foot of the mountain, so they could not approach God. In the Tabernacle and in the Temple the veil stood between them and God’s presence in the Holy of Holies. The Old Covenant not only did not bring men into God’s presence, it forbade them from trying to get there. Without full cleansing, complete forgiveness of sins, they were not qualified. But Jesus, so to speak, came down the mountain to the people and tore down the veil.