Oaths are serious business. We use them to enforce promises, make commitments, and swear allegiances. Generally, oaths are thought of as things we either have to do because of certain circumstances or things that can get us in trouble for violating them. But have you ever thought of an oath as a guarantee that is as valuable as gold, money, or precious objects?
Apparently, that's how Jesus' high priesthood functions through his humanity. Hebrews 7:20-22 states, "And it was not without an oath! Others became priests without any oath, but he became a priest with an oath when God said to him: 'The Lord has sworn and will not change his mind: "You are a priest forever."' Because of this oath, Jesus has become the guarantor of a better covenant" (NIV). When this passage talks about oaths, what it has in mind is the legitimacy of Jesus' high priesthood.
Most Old Testament priests were members of the Aaronic priesthood through the tribe of Levi. Nevertheless, they were not made priests by an oath; instead, they were permitted to serve as priests by God. Exodus 28:1 tells us that God selected Aaron and his descendants to be high priests in the sanctuary, but it doesn't say God installed them as priests or even swore that they would perpetually be priests. God invites them to serve before him as priests, but he never performs an oath to "make" them priests. Jesus, on the other hand, has received a perpetual oath from God that he will serve as a priest on our behalf according to the order of Melchizedek. Just as Melchizedek was a king and high priest for his people, so Jesus is a king and high priest for us.
All of this is pretty clear. But did you notice the last sentence in this passage? "Because of this oath, Jesus has become the guarantor of a better covenant." The Greek word for "guarantor" can either be translated as the one who enacts the covenant (namely, the guarantor) or the means by which the covenant is enacted (specifically, the guarantee). Both seem to be in mind for Hebrews 7:20-22. Jesus is the guarantee that we have a better covenant with God, because God has sworn such a covenant by declaring him a priest forever like Melchizedek. And Jesus is the guarantor of that covenant because, through his humanity, he knows what we need in order to be served and saved as our high priest.
Friends, take comfort in the fact that Jesus is both God's promise to us and God's fulfilment of that promise to us.