“For the law appoints as high priests men who are weak, but the word of the oath, which came after the law, appoints a Son, who has been perfected forever” (Heb. 7:28).
“The law” and “the word of the oath” are set up as contrasting covenants, contrasting approaches to God. “The word of the oath” connects back to verse 6:17 where the writer of Hebrews says the promise to Abraham was made with an oath.
The writer of Hebrews is taking us back to Abraham and the promise, back to the priesthood of Melchizedek, back before the law of Moses. He connects the promise with the new covenant and says Jesus is in the order of Melchizedek to help us see a certain continuity: The Son is heir to a more ancient promise, and He is in a more ancient priesthood.
Paul says it this way in Galatians: “The law, which came 430 years later, does not invalidate a covenant previously established by God and thus cancel the promise. For if the inheritance is based on the law, it is no longer based on the promise” (Gal. 3:17-18). He goes on to say that the law was given until the heir of the promise (Christ) came (Gal. 3:19).
The promise has primacy over the law. The promise came before the law, and the law was a temporary measure until the fulfillment of the promise.
Similarly, the order of Melchizedek has primacy over Levi. Hebrews 7:28 says the oath appointing the Son came after the law (uttered in Psalm 110). But the Son is appointed to the order of Melchizedek, which came before the law; Melchizedek ministered to Abraham. The levitical priesthood was temporary until Melchizedek’s successor arrived.
The writer of Hebrews helps us see that the new covenant was not new in the sense of novelty. Jesus, our High Priest, oversees a covenant that is the fulfillment of what was promised to Abraham.
The writer continues this point in the first six verses of chapter 8. Priests under the law offered gifts and sacrifices in the sanctuary Moses set up. But the sacrifices and sanctuary were “a copy and shadow of the heavenly things” (Heb. 8:5). He concludes that “Jesus has now obtained a superior ministry, and to that degree He is the mediator of a better covenant, which has been legally enacted on better promises” (Heb. 8:6).
In other words, Jesus belongs to a priesthood that precedes Levi, based on a promise that precedes the law, and offers a better sacrifice in the heavenly sanctuary Moses copied when he built the tabernacle. All of this means Christ’s covenant is superior.
On a side note: Talk of a new covenant in the New Testament begs a question: Is the new covenant something Jesus and His followers invented to discredit Judaism and objections to His messianic claims? This might be a fair question except for this fact: the need for a new covenant was raised by ministers under the old covenant and recorded in the Old Testament. The writer of Hebrews quotes Jeremiah’s declaration that “the days are coming, says the Lord, when I will make a new covenant with the house of Israel” (Heb. 8:8; Jer. 31:31).
Jesus did not make up the need for a new covenant. His assertion of a new covenant fulfilled Jeremiah’s prophecy. God identified the need for a better covenant through His prophet; God became man to act as priest, to offer a better sacrifice, and to minister in the heavenly sanctuary in order to enact the better covenant He predicted.