Verse 7 If that first had been faultless - This is nearly the same argument with that in Heb 7:11. The simple meaning is: If the first covenant had made a provision for and actually conferred pardon and purity, and given a title to eternal life, then there could have been no need for a second; but the first covenant did not give these things, therefore a second was necessary; and the covenant that gives these things is the Christian covenant.

The Lord sware and will not repent - Hence also it appears, that his is an unchangeable priesthood.

Heb 8:7

For if that first covenant had been faultless, then should no place have been sought for a second. "For" introduces this sentence as a reason for what has been already said; i.e. for a better covenant having been spoken of. The expression might be objected to by Hebrew readers as implying imperfection in the original Divine covenant. "Nay," says the writer, "it was imperfect, it was not faultless; for prophecy itself declares this." Should it be further objected that in the prophecy it is not the old covenant itself that is found fault with, but the people for not observing it, the answer would be that the remedy for their non-observance being the substitution of a new one that would answer its purpose better, some imperfection in the old one is implied. This is indeed the very point of this verse. If it be asked, further, how faultiness in the old covenant is compatible with the view of its Divine origin, the answer is abundantly supplied in St. Paul’s Epistles. His position constantly is that the Mosaic Law, though in itself "holy, just, and true," and adequate to its purpose, was still imperfect as a means of justification. It was but a temporary dispensation, with a purpose of its own, intervening between the original promise to Abraham and the fulfillment of that promise in Christ. Thus it is no derogation to itself or to its Author to charge it with "weakness and unprofitableness" for a purpose it was never meant to answer.

Verse 21. For those priests were made without an oath. The Levitical priests were set apart and consecrated without their office being confirmed to them by an oath on the part of God. They received it by regular descent, and when they arrived at a suitable age they entered on it of course. Jesus received his office by special appointment, and it was secured to him by an oath. The word rendered "oath" is, in the margin, "swearing of an oath". This is the proper meaning of the Greek word, but the sense is not materially varied. But this with an oath. This priest, the Lord Jesus, became a priest in virtue of an oath. The Lord sware. Heb 6:3. The reference here is to Ps 110:4 "The Lord hath sworn." and will not repent. That is, will not regret, or will not alter his mind through regret--for this is the meaning of the Greek word. (2) "an oath" "swearing of an oath" (e) "The Lord sware" Rom 5:2