“But He with an oath through the One who said to Him, ‘The Lord has sworn and will not change His mind, You are a priest forever.’ So much the more also Jesus has become the guarantee of a better covenant” (Heb 7:21-22). As a priest forever, Jesus is the guarantee of a better covenant than the one associated with the former priesthood.
“The former priests, on the one hand, existed in greater numbers, because they were prevented by death from continuing, but Jesus, on the other hand, because He continues forever, holds His priesthood permanently. Hence, also, He is able to save forever those who draw near to God through Him, since He always lives to make intercession for them” (Heb 7:23-25). Salvation through Jesus is complete because He is a priest forever.
“It is fitting that we should have such a high priest, holy, innocent, undefiled, separated from sinners and exalted above the heavens; who does not need daily, like those high priests, to offer up sacrifice, first for His own sins and then for the sins of the people, because this He did once for all when He offered up Himself. For the Law appoints high priests who are weak, but the word of the oath, which came after the Law, appoints a Son, made perfect forever” (Heb 7:26-28).
Letting the Bible speak for itself, we see that Jesus is a better priest and guarantees a better covenant. The Law, represented by the Levitical priesthood, was powerless and temporary. The new covenant is eternal and saves completely because its guarantor is Christ, the forever priest, who died once for all as a sacrifice for our sins and now intercedes for us before the Father. Calling Christ “holy, innocent, undefiled, and separated from sinners” highlights His sinlessness and no need for a sacrifice for Himself. His death was on our behalf and saves us completely and forever.
Continuing into chapter 8, “Now the main point in what we have said is this: we have such a high priest, who has taken His seat at the right hand of the throne of the Majesty in the heavens, a minister in the sanctuary and in the true tabernacle, which the Lord pitched, not man” (Heb 8:1-2).