20 And inasmuch as He was not made priest without an oath 21 (for they have become priests without an oath, but He with an oath by Him who said to Him: “The Lord has sworn And will not relent, ‘You are a priest forever According to the order of Melchizedek’ ”),
Another substantiation of the superiority of Christ’s priesthood is that it was ratified with a solemn oath by Jehovah.
7:20
And
The word “and” provides additional proof of the superiority of Christ’s priesthood.
inasmuch as He was not made priest without an oath
Jesus was made Priest by an oath from God. This oath is found in Psalm 110:4. It was an oath about the eternality of His priesthood.
7:21
The author inserted a following parenthesis to further confirm that the argument is from Scripture.
(for they have become priests without an oath,
Becoming a Levitical priest did not require the taking of an oath. Men became priests by being physically born into the tribe of Levi.
but He [the King-Priest] with an oath by [the Agent] Him who [God] said to Him [the King-Priest]:
This phrase is a strong affirmation (litotes); God swore by an irrevocable oath. The oath from Psalm 110:4 is by the Lord God to this person in line of the Davidic dynasty. In the second part, by trajectory, Christ the King-Priest was directly addressed by God “who said to Him.” On human standards it should have been Christ to swear the oath, but here He received an oath from God the Father.
God instituted Christ’s eternal priesthood with an “oath.” This was not true with the Levitical priesthood. God took the initiative to take the oath unilaterally.
“The Lord has sworn
Jehovah stands behind His appointments with a solemn oath.
And will not relent,
Jehovah’s oath to the Messiah was that it would be permanent; His oath is final. He never changes His mind or purpose. This guaranteed that His purpose for Christ as Priest would not change.
‘You are a priest forever According to the order of Melchizedek’ ”),
The Messiah’s oath is eternal (“forever”) according to Psalm 110:4 (He 7:3, 17).
PRINCIPLE:
Our Lord assumed a superior priesthood on our behalf.
APPLICATION:
Jesus is a royal Priest, making Him unique. This carries the idea that He was a timeless Priest. He was the only individual in His priesthood.
You said that God never changes His mind. But several places in the old testament it says that God repents or relents. Does that means He sometimes changes His mind?
JROCK, Note this study: https://versebyversecommentary.com/jonah/jonah-310/
The immutability of God means that He cannot change His essence. His existence is unalterable and He has no cause for Himself. He can never be better or worse than His essence.
This issue is anthropomorphic [characterizing God as man] or anthropopathic [characterizing God with man’s feelings]. God always has consistency with His attributes. When these changes appear, they do not appear in His essence, but in the execution of His essence. That is, if man repents from sin, then God will repent of judgment as in the case of Nineveh. God always acts consistently with His attributes when He see changes in man. God is always free to act according to His essence. Therefore, God is always at His best. No characteristic of God can ever change.