In Hebrews, the author talks about the mediator as a priest because in the language of Scripture, that is what a priest did on behalf of the people. The author of Hebrews develops that and points out that as the mediator, Jesus is the perfect priest. In addition, not only is Jesus the perfect priest, but he is also the perfect sacrifice. And because he’s the perfect sacrifice, it is therefore the final sacrifice. We need look for no other. Let me mention just a few of those important texts.
In Hebrews 7:26-28, the writer speaks of the comparison between the priests of his day and Jesus Christ. He points out that these Jewish priests had to first offer a sacrifice for themselves. Speaking of the annual Day of Atonement, if the high priest was going to take the sacrifice into the Holy of Holies, he had to first make a sacrifice for himself before he entered there because he, too, was a sinner. In these verses the author of Hebrews points out that Jesus didn’t have to do that, like other priests, because He was perfect to begin with. Also, in Hebrews 9:11-14, he points out that Christ not only is the perfect priest, but he is also the perfect sacrifice because he sacrifices himself. In addition, from Hebrews 10:11-14, he shows in a similar way that Jesus is also the final sacrifice and that now no more sacrifices need to be performed.
The third point is related to this, though less directly. Having first talked about our need for a mediator because of our sin, we then, secondly, saw that Jesus is the only one who can be this mediator and reconcile us to God through his atonement. The last point I want to make is that in light of these first two points we see that all of the essentials of Christianity are unified. You see, if there are diverse gods, there can be diverse religions and different ways to approach different gods. But if there is one God and one way to God, there can only be one true religion, and that is Christianity. In Ephesians 4:4-5, Paul is speaking of the unity that is ours in the faith, and he speaks of seven important unities: one body, one Spirit, one hope, one Lord, one faith, one baptism, one God and Father of all. This is why the church of Jesus Christ is one, even though there are differences in geography, time, or theological differences. All those who are believers in the Lord Jesus Christ as the only mediator between God and man are, nevertheless, one. The Westminster Confession of Faith talks about the church with this kind of unity in mind. Although there were different theological persuasions present regarding church government, the authors recognized their unity with all Christians because every true believer confesses that the Lord Jesus Christ is their Lord, and is the only mediator between God and man.